Potential for Saltwater Intrusion into the

Lower Tamiami Aquifer near Bonita Springs,

Southwestern Florida

Prepared in cooperation with the

South Florida Water Management District

By W. Barclay Shoemaker and K. Michelle Edwards

Tallahassee, Florida

2003

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 03-4262
U.S. Geological Survey

2010 Levy Avenue

Tallahassee, FL 32310

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

For additional information write to: Copies of this report can be purchased

from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Federal Center

Denver, CO 80225

888-ASK-USGS

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey.

Additional information about water resources in Florida is available on the Internet

at http://fl.water.usgs.gov
Contents III

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 2

Purpose and Scope ....................................................................................................................................................... 2

Description of Study Area ........................................................................................................................................... 2

Previous Studies........................................................................................................................................................... 5

Acknowledgments........................................................................................................................................................ 5

Hydrogeology of Southwestern Florida................................................................................................................................. 6

Lithology and Stratigraphy .......................................................................................................................................... 6

Hydrogeologic Units and Hydraulic Characteristics ................................................................................................... 7

Water Budget ............................................................................................................................................................... 15

Water Quality and Geochemistry................................................................................................................................. 20

Ground-Water Flow and Mechanisms of Saltwater Intrusion................................................................................................ 20

Lateral Encroachment .................................................................................................................................................. 22

Upward Leakage .......................................................................................................................................................... 22

Downward Leakage ..................................................................................................................................................... 24

Relict Seawater ............................................................................................................................................................ 25

Simulation of Saltwater Intrusion near Bonita Springs ......................................................................................................... 25

Modeling Approach ..................................................................................................................................................... 26

Code Selection ............................................................................................................................................................. 29

Constant-Density Ground-Water Flow Simulator.............................................................................................. 29

Variable-Density Ground-Water Flow Simulator .............................................................................................. 29

Spatial Discretization and Assignment of Aquifer Properties ..................................................................................... 30

Simulation of Predevelopment Conditions .................................................................................................................. 34

Boundary Conditions ......................................................................................................................................... 34

Recharge .................................................................................................................................................. 34

Rivers ....................................................................................................................................................... 35

No Flow ................................................................................................................................................... 35

Constant Heads and Concentrations ........................................................................................................ 37

Simulation Results ............................................................................................................................................. 37

Model Calibration to Typical, Modern, and Seasonal Stresses.................................................................................... 38

Boundary Conditions ......................................................................................................................................... 38

No Flow ................................................................................................................................................... 40

Constant Heads ........................................................................................................................................ 40

Rivers ....................................................................................................................................................... 40

General Heads.......................................................................................................................................... 41

Recharge .................................................................................................................................................. 41

Wells ........................................................................................................................................................ 42

Parameter Estimation ......................................................................................................................................... 42

Confidence Intervals .......................................................................................................................................... 49

Potential Movement of Saltwater to Equilibrium with Typical, Modern, and Seasonal Stresses................................ 52

Baseline Scenario............................................................................................................................................... 52

Mechanisms of Saltwater Intrusion ......................................................................................................... 53

Extent of Saltwater Intrusion ................................................................................................................... 54

Sensitivity Analyses........................................................................................................................................... 55

Mechanisms of Saltwater Intrusion ......................................................................................................... 56

Extent of Saltwater Intrusion ................................................................................................................... 57

Model Limitations........................................................................................................................................................ 60
IV Contents

Summary ............................................................................................................................................................................... 63

Selected References .............................................................................................................................................................. 64

Appendix: Monitoring Stations Used for this Study............................................................................................................. 69

PLATES

1. Maps showing boundary conditions for simulating the predevelopment distribution of saltwater................... pocket

2. Maps showing boundary conditions for calibrating the model to March and September 1996 

conditions and moving the predevelopment distribution of saltwater to equilibrium ........................................ pocket

FIGURES

1. Map showing location of the study area in southwestern Florida ............................................................................... 3

2. Maps showing major watersheds and physiographic features in the study area ......................................................... 4

3. Column showing generalized geologic and hydrogeologic units in the study area .................................................... 6

4. Histograms of the log of estimated horizontal hydraulic conductivity values of the water-table, 

lower Tamiami, and sandstone aquifers ...................................................................................................................... 8

5. Map showing lines of equal thickness of the rock units that comprise the water-table aquifer.................................. 9

6. Map showing spatial distribution of horizontal hydraulic conductivity in the water-table aquifer 

estimated from analyses of aquifer performance tests ................................................................................................ 10

7. Map showing lines of equal thickness of the rock units that comprise the lower Tamiami aquifer............................ 11

8. Map showing spatial distribution of horizontal hydraulic conductivity in the lower Tamiami aquifer 

estimated from analyses of aquifer performance tests ................................................................................................ 12

9. Map showing lines of equal thickness of the rock units that comprise the sandstone aquifer .................................... 13

10. Map showing spatial distribution of estimated horizontal hydraulic conductivity in the sandstone aquifer .............. 14

11. General hydrogeologic section showing hypothetical control volume and selected water-budget components 

within the study area.................................................................................................................................................... 15

12. Map showing average annual head difference between the lower Tamiami and sandstone aquifers in 1996............. 18

13. Bar graph showing average annual ground-water withdrawals from municipal supply facilities in 

Lee and Collier Counties, 1985-98.............................................................................................................................. 19

14. Bar graph showing average monthly ground-water withdrawals from municipal supply facilities in 

Lee and Collier Counties, 1995................................................................................................................................... 19

15. Scatter plot of historical trends in chloride concentration for selected monitoring wells near

Bonita Springs, 1968-98.............................................................................................................................................. 21

16. Maps showing lines of equal chloride concentration in the lower Tamiami aquifer near Bonita 

Springs over time......................................................................................................................................................... 22

17. Generalized cross section showing potential mechanisms of saltwater intrusion in the lower Tamiami 

aquifer near Bonita Springs......................................................................................................................................... 23

18. Bar graphs showing historical record of annual rainfall at Bonita Springs Utility rainfall monitoring 

station, 1943-54 and 1993-99...................................................................................................................................... 27

19. Graph of average monthly water levels from selected monitoring wells in the water-table and lower 

Tamiami aquifers near Bonita Springs ........................................................................................................................ 28

20. Three-dimensional diagram showing the distribution of bulk horizontal hydraulic conductivity in selected 

model cells................................................................................................................................................................... 32

21. Schematic of approach used to compute bulk hydraulic conductivities for each model cell using weighted 

arithmetic and harmonic averaging ............................................................................................................................. 33

22. Map showing average monthly water levels in the water-table and lower Tamiami aquifers during March 

and September 1996 .................................................................................................................................................... 36

23. Diagram showing three-dimensional view of the simulated predevelopment distribution of saltwater...................... 38

24. Map showing the approximate position of the simulated predevelopment saltwater interface at the 

base of the lower Tamiami aquifer .............................................................................................................................. 39

25. Bar graph showing total monthly pan evaporation from several monitoring stations near Bonita Springs ................ 42

26. Map showing areas with potentially large quantities of unmonitored ground-water pumpage .................................. 43

27. Bar graph showing composite-scaled sensitivities computed while calibrating the model to March and 

September 1996 conditions ......................................................................................................................................... 46
Contents V

28. Scatter plot showing comparison of observed and simulated water levels.................................................................. 48

29. Scatter plot showing comparison of weighted residuals and weighted simulated values ........................................... 49

30. Map showing simulated net recharge to the water-table aquifer during March 1996 ................................................. 50

31. Map showing simulated net recharge to the water-table aquifer during September 1996 .......................................... 51

32. Graph showing total salt mass in the model while moving the simulated predevelopment distribution 

of saltwater to equilibrium with calibrated March and September 1996 conditions................................................... 52

33. Map showing simulated potential extent of saltwater intrusion in the lower Tamiami aquifer from the 

baseline scenario near Bonita Springs......................................................................................................................... 53

34. Three-dimensional diagram of the saltwater interface and upconing plumes near equilibrium with 

calibrated March and September 1996 conditions in the water-table and lower Tamiami aquifers 

near Bonita Springs. .................................................................................................................................................... 54

35. Cross section A-A’ along row 40 of the model grid showing a mound of ground water between the 

cones of depression in the lower Tamiami aquifer and the saltwater interface ........................................................... 55

36. Map showing distribution of saltwater at the base of the lower Tamiami aquifer at the end of the baseline 

scenario, and two sensitivity analysis scenarios.......................................................................................................... 57

37. Bar graph showing sensitivity analysis of the extent of saltwater intrusion................................................................ 59

38. Graph showing average monthly rainfall from selected rainfall stations in southwestern Florida, 1909-99.............. 61

TABLES

1. Average 1996 water levels computed from selected monitoring wells in the lower Tamiami and 

sandstone aquifers ....................................................................................................................................................... 17

2. Description and primary use of modeling tasks .......................................................................................................... 25

3. Hydraulic conductivities used in the conceptual permeability model ........................................................................ 31

4. Observations used during model calibration to March and September 1996 conditions ........................................... 44

5. Estimates of unmonitored ground-water pumpage and evapotranspiration multipliers with 

confidence intervals computed by UCODE ............................................................................................................... 47

6. UCODE estimated parameter correlation coefficients ............................................................................................... 47

7. Changes made to baseline scenario parameters for a sensitivity analysis of the extent of saltwater 

intrusion in the lower Tamiami aquifer near Bonita Springs ...................................................................................... 58

CONVERSION FACTORS, ACRONYMS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply By To obtain

millimeter per year (mm/yr) 0.03937 inch per year

meter (m) 3.281 foot

meter per day (m/d) 3.281 foot per day

meter per year (m/yr) 3.281 foot per year

square meter (m2

) 10.76 square foot

cubic meter per day (m3/d) 35.31 cubic foot per day

cubic meter per second (m3

/s) 35.31 cubic foot per second

kilometer (km) 0.6214 mile

square kilometer (km2) 0.3861 square mile

centimeter (cm) 0.3937 inch

centimeter (cm) 3.281 x 10-2 foot

centimeter per day (cm/d) 3.281 x 10-2 foot per day

centimeter per year (cm/yr) 3.281 x 10-2 foot per year

kilogram per cubic meter (kg/m3

) 0.06243 pound per cubic foot
VI Contents

OTHER UNITS OF ABBREVIATION

ACRONYMS

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD of 1929); horizontal coordinate 

information is referenced to the North American Datum of 1983 (NAD83).

MHz megahertz

mg/L milligram per liter

ADAPS Automated Data Processing System

CSS composite-scaled sensitivities

DBHYDRO Hydrometeorologic Database (South Florida Water Management District)

DC direct current

HMOC hybrid method of characteristics

MAE mean absolute error

MOC method of characteristics

MMOC modified method of characteristics

NGVD National Geodetic Vertical Datum

NOAA National Oceanic and Atmospheric Administration

NWIS National Water Information System (U.S. Geological Survey database)

PSS prediction-scaled sensitivities

QWDATA Water Quality Database

SFWMD South Florida Water Management District

USGS U.S. Geological Survey

VCONT vertical conductance
Abstract 1

Potential for Saltwater Intrusion into the 

Lower Tamiami Aquifer near Bonita Springs, 

Southwestern Florida

By W. Barclay Shoemaker and K. Michelle Edwards

Abstract

A study was conducted to examine the 

potential for saltwater intrusion into the lower 

Tamiami aquifer beneath Bonita Springs in southwestern Florida. Field data were collected, and 

constant- and variable-density ground-water flow 

simulations were performed that: (1) spatially 

quantified modern and seasonal stresses, (2) identified potential mechanisms of saltwater intrusion, 

and (3) estimated the potential extent of saltwater 

intrusion for the area of concern.

MODFLOW and the inverse modeling 

routine UCODE were used to spatially quantify 

modern and seasonal stresses by calibrating a 

constant-density ground-water flow model to 

field data collected in 1996. The model was 

calibrated by assuming hydraulic conductivity 

parameters were accurate and by estimating 

unmonitored ground-water pumpage and 

potential evapotranspiration with UCODE. 

Uncertainty in these estimated parameters was 

quantified with 95-percent confidence intervals. 

These confidence intervals indicate more uncertainty (or less reliability) in the estimates of 

unmonitored ground-water pumpage than estimates of pan-evaporation multipliers, because of 

the nature and distribution of observations used 

during calibration. Comparison of simulated 

water levels, streamflows, and net recharge with 

field data suggests the model is a good representation of field conditions.

Potential mechanisms of saltwater intrusion 

into the lower Tamiami aquifer include: (1) lateral 

inland movement of the freshwater-saltwater 

interface from the southwestern coast of Florida; 

(2) upward leakage from deeper saline waterbearing zones through natural upwelling and 

upconing, both of which could occur as diffuse 

upward flow through semiconfining layers, conduit flow through karst features, or pipe flow 

through leaky artesian wells; (3) downward 

leakage of saltwater from surface-water channels; 

and (4) movement of unflushed pockets of relict 

seawater. Of the many potential mechanisms of 

saltwater intrusion, field data and variable-density 

ground-water flow simulations suggest that 

upconing is of utmost concern, and lateral 

encroachment is of second-most concern. This 

interpretation is uncertain, however, because the 

predominance of saltwater intrusion through 

leaky artesian wells with connection to deeper, 

more saline, and higher pressure aquifers was 

difficult to establish.

Effective management of ground-water 

resources in southwestern Florida requires an 

understanding of the potential extent of saltwater 

intrusion in the lower Tamiami aquifer near 

Bonita Springs. Variable-density, ground-water 

flow simulations suggest that when saltwater is 

at dynamic equilibrium with 1996 seasonal 

stresses, the extent of saltwater intrusion is 

about 100 square kilometers areally and 

70,000 hectare-meters volumetrically. The volumetric extent of saltwater intrusion was most sensitive to changes in recharge, ground-water 

pumpage, sea level, salinity of the Gulf of Mexico, and the potentiometric surface of the sandstone aquifer, respectively.
2 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

INTRODUCTION

Urbanization is proceeding rapidly along the 

coast of southwestern Florida, with cities and communities springing up from Fort Myers to Naples. In fact, 

the coast around Bonita Springs is one of the Nation’s 

fastest growing areas. The expanded population has 

resulted in increased public-supply withdrawals from 

the lower Tamiami aquifer that have lowered groundwater levels and reversed hydraulic gradients in the 

aquifer between Bonita Springs and the coastline. 

Long-term movement of saltwater into coastal aquifers 

is often attributed to declines in ground-water levels, 

thus limiting the future availability of potable-water 

supplies.

Saltwater intrusion is defined by Stewart (1999) 

as the mass transport of saline waters into zones previously occupied by fresher waters. This definition is 

broad because natural processes (such as sea level rise 

and drought) and anthropogenic processes (such as 

ground-water pumpage and construction of canals) can 

cause saltwater intrusion. Thus, the investigator is 

faced with a “classic” inverse problem because an 

observed distribution of saltwater could be explained 

equally well by one or more mechanisms of saltwater 

intrusion. Under these circumstances, multiple sources 

of information are useful to help identify the predominant mechanisms of saltwater intrusion, including 

ground-water levels and pumpage, geochemical data 

(such as chloride concentrations and strontium isotope 

analysis), surface and borehole geophysical data, 

research from previous studies, and numerical modeling tools.

 Water managers need a clear understanding of 

the saltwater intrusion process to ensure protection of 

the fresh ground-water resources in southwestern 

Florida. In 1999, the U.S. Geological Survey (USGS), 

in cooperation with the South Florida Water Management District (SFWMD), initiated a study to: (1) quantify modern (stresses with the desired characteristics) 

and seasonal stresses; (2) help identify potential mechanisms of saltwater intrusion; and (3) estimate the 

potential extent of saltwater intrusion in the lower 

Tamiami aquifer near Bonita Springs. Field data and 

numerical methods were used in this effort.

Purpose and Scope

The purpose of this report is to describe the 

results of numerical simulation procedures that represent the movement of saltwater in the lower Tamiami 

aquifer beneath Bonita Springs in southwestern 

Florida. Water-budget components were characterized 

and mathematically represented to help simulate 

ground-water flow and saltwater intrusion in the surficial aquifer system, which includes the lower Tamiami 

aquifer. A modular, three-dimensional, finite-difference, ground-water flow model was used to represent 

modern and seasonal stresses during March and September 1996 when ground-water levels are generally 

at their lowest and highest, respectively. Field data 

were collected and variable-density, ground-water 

flow simulations were performed to help identify 

mechanisms of saltwater intrusion of utmost concern 

and estimate the potential extent of saltwater intrusion 

in the lower Tamiami aquifer beneath Bonita Springs. 

The extent of saltwater intrusion was simulated from 

predevelopment distribution to dynamic equilibrium 

with calibrated March and September 1996 conditions.

Description of Study Area

The study area encompasses about 860 km2 in 

Lee and western Collier Counties (fig. 1), and is characterized by low topographic relief and a high water 

table. The physiographic provinces that comprise the 

study area (fig. 2) are the Gulf Coastal Lowlands, 

Caloosahatchee Incline, Desoto Plain, Southwestern 

Slope, Immokalee Rise, Everglades, Okeechobee 

Plain, Big Cypress Spur, and the Reticulate Coastal 

Swamps (Fernald and Purdum, 1998). The Immokalee 

Rise, a sandy ridge formed during high sea-level 

stands, occupies the highest part of the study area to 

the northeast (at an elevation of about 12 m) and 

borders the Caloosahatchee River and Big Cypress 

watersheds.

The Caloosahatchee River, Estero Bay, and Big 

Cypress watersheds are located in the study area 

(fig. 2). The Caloosahatchee River watershed extends 

from Lake Okeechobee to San Carlos Bay and passes 

through parts of Charlotte, Glades, Hendry, and Lee 

Counties and dips slightly into Collier County. The 

lower reaches of the watershed are characterized by a 

shallow bay, extensive seagrass beds, and sand flats.

The Estero Bay watershed (fig. 2) occupies central and southern Lee County and parts of northern 

Collier and western Hendry Counties. The principal 

surface-water features are Estero River, Spring Creek, 

Kehl Canal, Imperial River, and Cocohatchee River. 

The low gradients in these channels result in sluggish 

and tidally induced flow that is probably much greater 

than freshwater flow. Channel stages fluctuate 
Introduction 3

LEE

COUNTY

HENDRY

COUNTY

GLADES

COUNTY CHARLOTTE

COUNTY

COLLIER COUNTY

MONROE COUNTY

MIAMI-DADE COUNTY BROWARD COUNTY PALM BEACH COUNTY

Tamiami Canal

Cocohatchee

River

Estero

River

Spring

Creek

Imperial

River Kehl Canal

Estero

Bay

San

Carlos

Bay

Lake

Okeechobee

Ca a loos hatchee River

GULF OF MEXICO

Bonita

Springs

Belle

Meade

Fort

Myers Lehigh

Acres

Naples

Immokalee

Cape

Coral

Sanibel Island

846

41

41

82

884

75

29

29

Trail Tamiami

Alligator Alley

EXPLANATION

STUDY AREA

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

0

0

15 MILES

15 KILOMETERS

5 10

5 10

25°30 ́

25°45 ́

26°15 ́

26°00 ́

26°30 ́

26°45 ́

82°15 ́ 82°00 ́ 81°45 ́ 81°30 ́ 81°15 ́ 81°00 ́ 80°45 ́

N Branch

Estero

S Branch

Estero

Imperial

L-738

C-1083

C-54

C-391

C-392

C-460

C-492

C-496

C-690

C-951

C-953

C-968

C-1004 C-988

C-1071

L-730

L-1403

L-1995

L-2195

L-2550

L-5724

L-5725

“mud hole”

(Kohout, 1979)

L-5726

L-5727

C-1082

L-5722

L-5723

ROAD

CANAL OR RIVER

CONTROL STRUCTURE

WELL LOCATION AND NUMBER

FLOW GAGING STATION LOCATION

AND NAME

Imperial

C-496

Figure 1. Location of the study area in southwestern Florida.
4 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida CALOOSAHATCHEE RIVER WATERSHED ESTERO BAY WATERSHED BIG CYPRESS WATERSHED CANAL OR RIVER

ROAD CANAL 

ROAD

GULF COASTAL LOWLANDSCALOOSAHATCHEE INCLINEDESOTO PLAINSOUTHWESTERN SLOPE

OKEECHOBEE PLAIN

IMMOKEVERGBIG CY

RETICU

00

15 MILES

15 KILOMETERS

5 10

5 10

LEE

COUNTY

LEE

COUNTY

H

C

HENDRY

COUNTY

GLADES

COUNTY

GLADES COUNTY

CHARLOTTE

COUNTY

CHARLOTTE

COUNTY

COLLIER COUNTY 

Ten Thousand Islands MONROE COUNTY

MIAMI-DADE COUNTY

BROWARD COUNTY

PALM BEACH COUNTY

Tamiami Canal

Lake

Okeechobee

San

Carlos

Bay

San

Carlos

Bay

Ca a loos hatchee River

Ca a loos hatchee River

GULF OF MEXICO

GULF OF MEXICO

Bonita

Springs

Bonita

Springs

Belle

Meade

Belle

Meade

Fort

Myers

Fort

Myers Lehigh

Acres

Lehigh

Acres

Naples Naples

Immokalee Immokalee

CapeCoral

CapeCoral

Sanibel Island Sanibel Island

Trail

Trail

Tamiami

Tamiami

Alligator Alley 

25°30 ́ 25°30 ́25°45 ́ 25°45 ́26°15 ́ 26°15 ́26°00 ́ 26°00 ́26°30 ́ 26°30 ́26°45 ́ 26°45 ́

82°15 ́ 82°00 ́ 81°45 ́ 81°30 ́ 81°15 ́ 81°00 ́ 80°45 ́ 82°15 ́ 82°00 ́ 81°45 ́ 81°30 ́ 81°1

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator projection, zone 17

41

41

846

82

884

29

29 

41

41

846

82

884

29

29 75

EXPLANATION EXPLANATION

A 

Figure 2. (A) Major watersheds (South Florida Water Management District, 1989), and (B) physiographic features (modified from Fernastudy area. The Big Cypress watershed includes the Corkscrew Swamp Sanctuary northeast of Naples, Florida.
Introduction 5

seasonally with the highest and lowest levels occurring 

during summer and winter, respectively. The flora and 

fauna of the Estero Bay watershed are varied and 

abundant.

The Big Cypress watershed (fig. 2) drains a 

wide area through a large network of man-made canals 

and natural sloughs in southern Lee and Hendry Counties and western Collier County. A variety of grasses, 

shrubs, and small cypress trees dominate the Big 

Cypress watershed, which includes the Corkscrew 

Swamp Sanctuary located northeast of Naples. South 

of Naples, freshwater sloughs and rivers mix with Gulf 

of Mexico bays and tidal creeks.

Development in the study area is primarily 

urban and agricultural. Urban development is greatest 

along the coast west of I-75 (Interstate 75) in Lee and 

Collier Counties. Agricultural development, located 

between wetland systems, increases to the east and 

south in the study area. The undeveloped regions consist of flatwoods, sloughs, swamps, and estuaries. Predevelopment sheetflow in the study area probably was 

slow due to the vegetation, geomorphology, and low 

land-elevation gradients. Canals and roads resulting 

from urban and agricultural development have nearly 

eliminated the natural sheetflow across the land surface and altered the natural drainage patterns by concentrating stormwater runoff in canals and ditches.

Previous Studies

Many studies of saltwater intrusion have been 

conducted over the years, and a comprehensive record 

can be found in Bear and others (1999). Two “classic” 

studies were completed by Badon-Ghyben (1888) and 

Herzberg (1901) in which the depth of the saltwater 

interface below sea level was predicted to be 40 times 

the freshwater head above sea level in the aquifer of 

interest, given hydrostatic conditions in a homogeneous unconfined aquifer with a seaward sloping 

water-table surface. Kohout (1964) recorded that the 

saltwater front in the Biscayne aquifer in southeastern 

Florida is as much as 13 km seaward of the position 

predicted by the Ghyben-Herzberg equation due to 

ground-water circulation patterns near the saltwater 

interface. Later research of saltwater intrusion in 

southeastern Florida was accomplished by Hull and 

Meyer (1973), Klein and Waller (1985), Klein and 

Ratzlaff (1989), Sonenshein and Koszalka (1996), 

Merritt (1996), Sonenshein (1997), Konikow and 

Reilly (1999), and Langevin (2001). Merritt (1996) 

used a sensitivity analysis of numerical simulations to 

address the importance of properties and processes 

affecting the saltwater interface in Broward County 

along the southeastern coast of Florida.

On the southwestern coast of Florida, Klein 

(1954) studied saltwater intrusion in coastal areas near 

Naples. In the 1960’s, field studies by McCoy (1962), 

Cooper (1964), Henry (1964), and Glover (1964) 

advanced the knowledge and understanding of groundwater movement near the saltwater interface in southern Florida. Sherwood and Klein (1963) mapped a 

salinity plume beneath the Caloosahatchee River, 

whose source was a leaky well with connection to the 

deeper, more saline, and higher pressure Floridan aquifer system. Kohout (1979) used remote sensing to map 

a geothermal submarine ground-water spring off the 

coast of Bonita Springs, informally known as the 

“mudhole” by local fisherman. Stewart and others 

(1982) used direct-current (DC) resistivity soundings 

to map the saltwater interface near Belle Meade south 

of the study area.

Acknowledgments

 Technical support for this project was provided 

by Chris Langevin and Keith Halford of the USGS, 

Mark Stewart of the University of South Florida 

Department of Geology, and Akin Owosina and Terry 

Bengtsson of the SFWMD. Terry Bengtsson and Lee 

Worst from the SFWMD helped delineate areas near 

Bonita Springs where unmonitored ground-water 

pumpage is likely occurring. Pamela Telis helped 

design, distribute, and compile results from a survey 

designed to estimate unmonitored ground-water 

pumpage. Several colleagues reviewed the initial 

drafts of this report, including Terry Bengtsson and 

Rama Rani, and the following USGS employees: 

Barbara Howie, Eric Swain, Chris Langevin, Melinda 

Wolfert, Richard Krulikas, Ward Sanford, Gary 

Mahon, Eve Kuniansky, Michael Deacon, Roy 

Sonenshein and David Schmerge. Fernando MirallesWilhelm of the University of Miami and Eric Swain of 

the USGS provided useful insight into the sensitivity 

behavior of saltwater intrusion simulations. Rhonda 

Howard and Barbara Howie provided many suggestions that significantly improved the clarity and content of the report. Kimberly Swidarski prepared the 

illustrations. USGS employee Haymeli Castillo, is 

recognized for her constant support, patience, and 

guidance.
6 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

HYDROGEOLOGY OF SOUTHWESTERN 

FLORIDA

Southern Florida is underlain by rocks of 

Cenozoic age to a depth of about 1,525 m. These rocks 

principally are carbonates (limestone and dolostone), 

with minor amounts of evaporites (gypsum and anhydrite) in the lower part of the section and clastics (sand 

and clay) in the upper part. The movement of ground 

water from inland areas to the ocean (and the reverse) 

occurs primarily through the carbonate rocks (Meyer, 

1989, p. G5). This section of the report describes the 

hydrogeologic framework of the study area including 

the lithology, stratigraphy, hydrogeologic units, and 

hydraulic characteristics. Also discussed are the waterbudget components, which used in conjunction with 

the hydrogeologic framework, are necessary to understand the processes that affect ground-water flow. 

Finally, water-quality and geochemical data collected 

from the lower Tamiami aquifer near Bonita Springs 

are used to examine salinity. The locations of monitoring stations for data used in the study are presented in 

the appendix.

Lithology and Stratigraphy

Geologic units in southwestern Florida consist, in 

ascending order, of the Suwannee Limestone of 

Oligocene age, Hawthorn Group of Oligocene to 

Pliocene age, Tamiami Formation of Pliocene age, and 

undifferentiated sediments of Holocene to Pleistocene 

age (fig. 3). The Suwannee Limestone is composed of 

fossiliferous, calcarenitic limestone with minor amounts 

of quartz sand. The thickness of the limestone varies 

widely, but commonly is greater than 100 m in Lee and 

Collier Counties. The basal Suwannee Limestone generally contains fine-grained, phosphatic, clastic material 

with interbeds of micrite and clay (Reese, 2000).

The Hawthorn Group is divided into the Arcadia 

Formation and the Peace River Formation (fig. 3). The 

Arcadia Formation, which unconformably overlies the 

Suwannee Limestone, consists of fine-grained carbonate sediments as well as sandy limestone, shell beds, 

dolomite, phosphatic sand and carbonate, sand, silt, 

and clay. The predominantly clastic Peace River Formation has a highly irregular erosional, and karstic 

surface. The contact with the overlying Tamiami 

Formation appears to be unconformable in some areas, 

Series Geologic

Unit

Hydrogeologic unit

HOLOCENE

TO

PLEISTOCENE UNDIFFERENTIATED

SURFICIAL

AQUIFER SYSTEM

WATER-TABLE

AQUIFER

CONFINING BEDS

LOWER TAMIAMI

AQUIFER

CONFINING UNIT

SANDSTONE

AQUIFER

CONFINING UNIT

MID-HAWTHORN

AQUIFER

INTERMEDIATE AQUIFER

SYSTEM

CONFINING UNIT

LOWER HAWTHORN

PRODUCING ZONE

UPPER

FLORIDAN

AQUIFER

MIOCENE

PLIOCENE

OLIGOCENE

TAMIAMI

FORMATION

HAWTHORN GROUP

PEACE

RIVER

FORMATION

ARCADIA

FORMATION

SUWANNEE

LIMESTONE

FLORIDAN AQUIFER

SYSTEM

Figure 3. Generalized geologic and hydrogeologic units in the study area. 

Modified from Reese (2000).
Hydrogeology of Southwestern Florida 7

but indistinct in other areas. The siliciclastic sediments 

of the Peace River Formation consist of interbedded, 

fine- to coarse-grained quartz sand, quartz silt, gravel, 

clay, carbonate, and phosphatic sand (Reese, 2000).

The Tamiami Formation overlies the Peace River 

Formation and consists of varying amounts of silt, 

sandy clay, micritic limestone, sandy and shelly limestone, calcareous sandstone, and quartz sand. The lithology of the Tamiami Formation varies greatly because of 

the complex nature of the depositional environment. 

The limestone is well indurated to unindurated, slightly 

phosphatic, variably sandy, and fossiliferous. The sand 

facies varies from well sorted, clean sand with abundant 

shells and traces of silt-size phosphate, to clayey sand 

with sand-size phosphate, clay-size carbonate in the 

matrix, and abundant well-preserved mollusk shells 

(Knapp and others, 1986; Reese, 2000).

The undifferentiated sediments of Holocene to 

Pleistocene age overlie the Tamiami Formation at land 

surface (Reese, 2000). These deposits mainly consist 

of quartz sand with minor amounts of shell and clay, 

and contain interfingering limestones, sandstones, and 

shell beds. With increasing elevation inland, the sand 

becomes thicker and less calcareous. The sand facies 

varies from fine to coarse grained, nonindurated to 

poorly indurated, and nonclayey to slightly clayey. 

Included in this group are marine terrace sediments, 

aeolian sand dunes, fluvial deposits, freshwater carbonates, peats, and clay beds.

Hydrogeologic Units and Hydraulic 

Characteristics

The principal aquifer systems in southwestern 

Florida are, in descending order, the surficial, intermediate, and Floridan aquifer systems (fig. 3). The focus 

of this report is the water-table and lower Tamiami 

aquifers of the surficial aquifer system and the sandstone aquifer of the upper part of the intermediate 

aquifer system. The confining beds of the Tamiami 

Formation and the upper confining unit of the Peace 

River Formation also are considered because they vertically “straddle” the lower Tamiami aquifer. For simplicity, these semiconfining units are called the 

Tamiami confining beds and the upper Hawthorn confining unit (Wedderburn and others, 1982) in this 

report. More detailed information on the water-bearing 

and confining units in southwestern Florida is presented in Reese (2000).

Maps are used in this section to show estimated 

hydraulic conductivities and depict the lines of equal 

thickness of the rock units that comprise the watertable, lower Tamiami, and sandstone aquifers. Maps 

showing lines of equal thicknesses were constructed 

by the South Florida Water Management District 

(1994). Histograms (fig. 4) and maps showing 

estimated hydraulic conductivities were developed 

using well construction reports and previous aquifer 

performance test data (Knapp and others, 1986; 

Montgomery, 1988). Many previously conducted aquifer performance tests were reanalyzed to assess reliability and to “fill in data gaps” (K.M. Edwards, U.S. 

Geological Survey, written commun., 2000). Horizontal hydraulic conductivity values (in meters per day) 

were computed by dividing transmissivity estimates by 

the thickness of the aquifer at the location of the aquifer performance test. Numerous estimates of hydraulic 

characteristics were compiled from various sources. 

Hydraulic conductivity was assumed to be log-normally distributed when computing permeability averages and standard deviations (fig. 4).

In southwestern Florida, the water-table aquifer 

is present in the undifferentiated deposits of Holocene 

to Pleistocene age and the upper part of the Tamiami 

Formation (fig. 3). The lines of equal thickness of the 

rock units that comprise the water-table aquifer range 

from 6 m (about 20 ft) in Hendry County south of the 

Caloosahatchee River to 30 m (about 100 ft) in northern Collier County along the border with Lee and Hendry Counties (fig. 5). The spatial distribution of 

estimated horizontal hydraulic conductivity in the 

water-table aquifer (fig. 6) reflects the complex heterogeneity of this characteristic in the study area. In log 

space (fig. 4A), the mean and standard deviation of 

244 log estimates of horizontal hydraulic conductivity 

in the water-table aquifer were 2.16 and 0.6, respectively. In parameter space, this translates to a range of 

about 40 to 600 m/d, which is one standard deviation 

from the mean.

The water-table aquifer is underlain by rocks 

and sediments that form the confining beds of the 

Tamiami Formation (fig. 3). The thickness of these 

Tamiami confining beds ranges from 0 to about 15 m 

(about 0 to 50 ft) in Lee, Collier, and Hendry Counties 

(South Florida Water Management District, 1994). The 

beds, which pinch out in the central and northern parts 

of Lee County and northwestern Hendry County, are 

thickest in localized areas near Bonita Springs in 

northern and eastern Collier County, and in central and 
8 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

WATER-TABLE AQUIFER

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

LOWER TAMIAMI AQUIFER

SANDSTONE AQUIFER

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

COUNT COUNT COUNT

LOG HORIZONTAL HYDRAULIC CONDUCTIVITY BINS

LOG HORIZONTAL HYDRAULIC CONDUCTIVITY BINS

LOG HORIZONTAL HYDRAULIC CONDUCTIVITY BINS

0

1

2

3

4

5

6

7

8

A

B

C

Figure 4. Estimated horizontal 

hydraulic conductivity values of 

the (A) water-table, (B) lower 

Tamiami, and (C) sandstone 

aquifers. The values of hydraulic 

conductivity are in meters per day.
Hydrogeology of Southwestern Florida 9

southern Hendry County. Sparse data are available to 

describe the distribution of vertical hydraulic conductivity in the Tamiami confining beds. Montgomery 

(1988) reports a value of 0.004 m/d for vertical 

hydraulic conductivity based on aquifer performance 

tests. In a reanalysis of aquifer performance tests 

(K.M. Edwards, U.S. Geological Survey, written commun., 2000), four reliable values of the vertical 

hydraulic conductivity of the Tamiami confining beds 

each equaled about 0.004 m/d.

LEE

COUNTY

HENDRY

COUNTY

GLADES

COUNTY

CHARLOTTE

COUNTY

COLLIER COUNTY

MONROE COUNTY

MIAMI-DADE COUNTY BROWARD COUNTY PALM BEACH COUNTY

846

41

41

82 884

75 29

29

Trail

Tamiami

Alligator Alley

Bonita

Springs

Fort

Myers Lehigh

Acres

Naples

Immokalee

Immokalee

Rise

Cape

Coral

Sanibel Island

Tamiami Canal

Lake

Okeechobee

Ca a loos hatchee River

GULF OF MEXICO

40

40

40

40

40

40

20

20

40

40

20

20

20

40

60

60

60

60

40

60

80

80

80

100

0

0

15 MILES

15 KILOMETERS

5 10

5 10

25°30 ́

25°45 ́

26°15 ́

26°00 ́

26°30 ́

26°45 ́

82°15 ́ 82°00 ́ 81°45 ́ 81°30 ́ 81°15 ́ 81°00 ́ 80°45 ́

ROAD

STUDY AREA BOUNDARY

LINE OF EQUAL THICKNESS--Shows

thickness, in feet. Interval is 20 feet

Base from U.S. Geological Survey digital data, 1:100,000 EXPLANATION Universal Transverse Mercator projection, zone 17

CANAL OR RIVER

Figure 5. Lines of equal thickness of the rock units that comprise the water-table aquifer. From the South Florida 

Water Management District (1994).
10 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Underlying the confining beds, the lower 

Tamiami aquifer, a major water-producing unit 

within much of the study area, generally lies within 

the lower part of the Tamiami Formation of Pliocene 

age (fig. 3). In some places, the lower Tamiami 

aquifer includes unconsolidated quartz sand of late 

Miocene age (Knapp and others, 1986; Weedman 

and others, 1997; Edwards and others, 1998). The 

lines of equal thickness of the rock units that 

comprise the lower Tamiami aquifer ranges from 

0 to 55 m (about 0 to 180 ft) in Lee, Henry, and 

Collier Counties (fig. 7). The aquifer, which is not 

present in the central and northern parts of Lee 

County and in northwestern Hendry County, is 

thickest in localized areas southeast of Bonita Springs 

(fig. 7).

900

90 90

90

700 2,000

2,000

2,000

2,000

2,000

900

400

900

900

500

400 1,000

1,000

1,000

1,000

1,000

1,000

1,000

900

1,000

300 1,000

300

300

300 300

300

300

300

50

50

50

300

30

30

200

200

200

200

200

0

0

6 MILES

6 KILOMETERS

2 4

2 4

81°45 ́ 81°30 ́

26°30 ́

26°15 ́

26°00 ́

GULF OF MEXICO

Naples

LEE

COUNTY

COLLIER

COUNTY

Tamiami Canal

Trail

Tamiami

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

ROAD

POINT VALUE OF ESTIMATED HYDRAULIC

CONDUCTIVITY--Shown in meters per day.

Multiple values indicate that more than one

test was performed at the site

CANAL OR RIVER

EXPLANATION

200

900

200

200

200

200

200

200

200

200

200

200

200

200

200

200

200

800

200

200

600

600

60

60

60

100

100

100

100

100

100

300

300

700 500 70

70

400

400

9

9

9 50

30

6 100

Alligator Alley 75

Belle

Meade

846

41

Cocohatchee

River

Imperial

River

Estero

River

Figure 6. Spatial distribution of horizontal hydraulic conductivity in the water-table aquifer estimated from analyses of 

aquifer performance tests.
Hydrogeology of Southwestern Florida 11

Estimates of horizontal hydraulic conductivity 

in the lower Tamiami aquifer seem to have a bimodal 

log K distribution (fig. 4B). Results from this and previous research (Meeder, 1979) suggest that the lower 

Tamiami aquifer is highly permeable where coral reef 

facies are present (fig. 8). In the coral reef facies 

mapped by Meeder (1979), the mean and standard 

deviation of eight log estimates of horizontal hydraulic 

conductivity (fig 4B) were 2.11 and 0.4, respectively. 

In parameter space, this translates to a range of about 

50 to 300 m/d, which is one standard deviation from 

the mean. In places within the lower Tamiami aquifer 

LEE

COUNTY HENDRY

COUNTY

GLADES COUNTY

CHARLOTTE

COUNTY

COLLIER COUNTY

MONROE COUNTY

MIAMI-DADE COUNTY BROWARD COUNTY PALM BEACH COUNTY

LATE

Q

RAL EXTENT OF THE A UIFER

25°30 ́

25°45 ́

26°15 ́

26°00 ́

26°30 ́

26°45 ́

82°15 ́ 82°00 ́ 81°45 ́ 81°30 ́ 81°15 ́ 81°00 ́ 80°45 ́

ROAD

STUDY AREA BOUNDARY

LINE OF EQUAL THICKNESS--Shows

thickness, in feet. Interval is 20 feet

EXPLANATION

CANAL OR RIVER

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

0 15 MILES 5 10

Tamiami Canal

Lake

Okeechobee

Ca a loos hatchee River

GULF OF MEXICO

80

80

80

80

80

80

60

60

60

60

80

100

120

100

20

20

20

40

40

120

140 140

160

140

180

846

41

41

82 884

75

29

29

Trail

Tamiami

Alligator Alley

0 15 KILOMETERS 5 10

Bonita

Springs

Fort

Myers Lehigh

Acres

Naples

Immokalee

Cape

Coral

Sanibel Island

Figure 7. Lines of equal thickness of the rock units that comprise the lower Tamiami aquifer. From the South 

Florida Water Management District (1994).
12 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

where coral reef facies are not present, the mean and 

standard deviation of 136 log estimates of horizontal 

hydraulic conductivity (fig. 4B) were 1.8 and 0.4, 

respectively. In parameter space, this translates to a 

range of about 30 to 200 m/d, which is one standard 

deviation from the mean.

The lower Tamiami aquifer is underlain by rocks 

and sediments that form the upper confining unit of the 

Hawthorn Group (fig. 3). This unit, called the upper 

Hawthorn confining unit in this report, thins and may be 

absent near Bonita Springs (Wedderburn and others, 

1982). Limited data are available to describe the 

200

40

90

90

100 60 200 50

600

40

50

50

200

50

30

60

40 70 61 200

200 39 200

200 30 200 70 28 30

80 50

50 100

40

50

50 60

60 300

300 40

300

50

70

40 70

90

80

70

40 200

80 10

80

80

90

10

50

81°45 ́ 81°30 ́

26°15 ́

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

Naples

Trail

Tamiami

EXPLANATION

LEE COUNTY

COLLIER COUNTY

GULF OF MEXICO

Tamiami Canal

ROAD

CANAL OR RIVER

APPROXIMATE LOCATION OF CORAL REEF FACIES--Based on Meeder (1979)

POINT VALUE OF ESTIMATED HYDRAULIC CONDUCTIVITY--Shown in meters

per day. Multiple values indicate that more than one test was performed at the site

80

80

0

0

6 MILES

6 KILOMETERS

2 4

2 4

Alligator Alley 75

41

Cocohatchee

River

Imperial

River

Estero

River

Bonita

Springs

75

846

Figure 8. Spatial distribution of horizontal hydraulic conductivity in the lower Tamiami aquifer 

estimated from analyses of aquifer performance tests.
Hydrogeology of Southwestern Florida 13

distribution of vertical hydraulic conductivity in the 

upper Hawthorn confining unit. Montgomery (1988) 

reports a value of 0.003 m/d for vertical hydraulic 

conductivity based on aquifer performance tests. This 

value was used by Bower and others (1990) and Bennett 

(1992) in numerical models for parts of the study area.

The sandstone aquifer underlies the upper Hawthorn confining unit in the intermediate aquifer system 

(fig. 3). The lines of equal thickness of the rock units 

that comprise the sandstone aquifer range from 0 to 

33 m (0 to about 110 ft) in Lee, Henry, and Collier 

Counties (fig. 9). The sandstone aquifer seems to be 

LEE

COUNTY

HENDRY

COUNTY

GLADES COUNTY

CHARLOTTE

COUNTY

COLLIER COUNTY

MONROE COUNTY

MIAMI-DADE COUNTY BROWARD COUNTY PALM BEACH COUNTY

Bonita

Springs

Fort

Myers Lehigh

Acres

Naples

Immokalee

Sanibel Island

846

41

41

82

884

75

29

29

Trail

Tamiami

Alligator Alley

AQUIFERABSENT

25°30 ́

25°45 ́

26°15 ́

26°00 ́

26°30 ́

26°45 ́

82°15 ́ 82°00 ́ 81°45 ́ 81°30 ́ 81°15 ́ 81°00 ́ 80°45 ́

ROAD

STUDY AREA BOUNDARY

LINE OF EQUAL THICKNESS--Shows

thickness, in feet. Interval is 20 feet

LATERAL EXTENT OF AQUIFER

EXPLANATION

CANAL OR RIVER

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

0

0

15 MILES

15 KILOMETERS

5 10

5 10

Tamiami Canal

Lake

Okeechobee

Ca a loos hatchee River

GULF OF MEXICO

10

10

10

10

10

30

30

30

30

30

30

50

50

50

50

50

70

70

70

70

70

70

90

90

90

90

110

110

110

Figure 9. Lines of equal thickness of the rock units that comprise the sandstone aquifer. From the South Florida 

Water Management District (1994).
14 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

thickest in localized areas in northern Collier County, 

southeastern Lee County, and northeastern Lee County 

and is absent in southeastern Collier County, northeastern Hendry County, and in parts of northwestern 

Lee County (fig. 9). The spatial distribution of 

estimated horizontal hydraulic conductivity in the 

sandstone aquifer (fig. 10) reflects the complex heterogeneity of this characteristic in the study area. In log 

space (fig. 4C), the mean and standard deviation of 

25 log estimates of horizontal hydraulic conductivity 

in the sandstone aquifer were 1.26 and 0.4, respectively. In parameter space, this translates to a range of 

about 10 to 50 m/d, which is one standard deviation 

from the mean.

10

6

30

30

200

200

200

30

20

30 7

20

10

20

30 20

20

20

20

60

40

60

70

70

40

20

10

2

20

10

10

9 10

10

10

20

4

10

10 6

30

30

6 30

10

60

8

8

0

0

6 MILES

6 KILOMETERS

2 4

2 4

81°45 ́ 81°30 ́

26°15 ́

26°30 ́

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

ROAD

CANAL OR RIVER

EXPLANATION

GULF OF MEXICO

8

8

POINT VALUE OF ESTIMATED

HYDRAULIC CONDUCTIVITY--

Shown in meters per day. Multiple

values indicate that more than one

test was performed at the site

846

75

41

LEE

COUNTY

COLLIER COUNTY

Cocohatchee

River

Imperial

River

Estero

River

Bonita

Springs

Figure 10. Spatial distribution of estimated horizontal hydraulic conductivity in the sandstone aquifer.
Hydrogeology of Southwestern Florida 15

Water Budget

This section describes components of the water 

budget that may be affecting saltwater movement in 

the water-table and lower Tamiami aquifers near 

Bonita Springs (fig. 11). Equation 1 provides a qualitative understanding of how these components interact. 

The terms are not summed together in this analysis; 

therefore, the units of each component are sometimes 

reported differently depending upon the source of 

information.

, (1)

where

P is precipitation; 

I is irrigation;

ET is evapotranspiration;

D is net deep leakage between the sandstone 

aquifer and the lower Tamiami aquifer;

QS is submarine ground-water discharge to the 

Gulf of Mexico,

QP is pumpage from the water-table and lower 

Tamiami aquifers;

Q is stream discharge, which is composed of surface runoff, Qro, and baseflow, QB; 

Quk is other or unknown components of the water 

budget that may be affecting saltwater 

movement; and

∆S is change in storage, which is assumed to be 

negligible over the long term.

Precipitation (P) and irrigation (I) that exceed 

evapotranspiration (ET) and runoff (Qro) can recharge 

the water table if it infiltrates land surface and overcomes capillary forces in the unsaturated zone. This 

recharge is generally called net recharge. Krulikas 

and Giese (1995) estimate net recharge in the study 

area to range from 1.5 to 22.9 cm/yr, using chloride 

concentration ratios and flow-tube analysis methods. 

They further indicate that net recharge rates greater 

than 22.9 cm/yr could be induced by lowering the 

water table through ground-water withdrawals or 

seepage to canals.

Precipitation

or irrigation

(P+I)

Saltwater

EXPLANATION

Freshwater or brackish water

WATER-TABLE AQUIFER

TAMIAMI CONFINING BEDS

Pumpage

(Q ) P

Submarine

ground-water

discharge

Streamflow discharge

(composed of surface

runoff and baseflow)

(Q)

Evapotranspiration

(ET)

GULF OF MEXICO

LOWER TAMIAMI AQUIFER

Deep leakage

Leakage

Deep leakage

30

-30

-60

NGVD 29

30

NGVD 29

-30

-60

METERS METERS

IMMOKALEE

WEST EAST

BONITA

SPRINGS

SANDSTONE AQUIFER

UPPER HAWTHORN CONFINING UNIT

(Q ) S

(D ) up

(D ) down

Figure 11. Hypothetical control volume and selected water-budget components within the study area. Dup is deep 

upward leakage and Ddown is deep downward leakage. NGVD 29 is National Geodetic Vertical Datum of 1929.

P I ET – – D QS – QP – – Q Quk + – = ∆S ≅ 0
16 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Net deep leakage, D, represents the difference 

between upward, Dup,

 and downward leakage, Ddown, 

that occurs naturally between the lower Tamiami and 

sandstone aquifers. This leakage results from the 

semiconfining nature of the upper Hawthorn confining 

unit and the difference in head between the aquifers 

(fig. 11). Deep leakage was inferred between the 

lower Tamiami and sandstone aquifers using a map of 

the head difference between these two units. Average 

1996 water levels were computed from water levels 

measured in 45 USGS monitoring wells in the lower 

Tamiami aquifer, and from 30 USGS monitoring wells 

in the sandstone aquifer (table 1). Average water levels 

were plotted and contoured and spatially interpolated 

to a common grid. The difference in interpolated head 

between the lower Tamiami and sandstone aquifers 

was then computed. This approach has many assumptions including: (1) water-level contours in the lower 

Tamiami aquifer are vertical; (2) water-level contours 

in the sandstone aquifer are vertical; and (3) the head 

difference, as calculated from the water-level contours, 

occurs directly within the upper Hawthorn confining 

unit.

In 1996, head differences between the lower 

Tamiami and sandstone aquifers ranged from -2.0 to 

2.6 m (fig. 12). A negative sign suggests leakage is 

upward into the lower Tamiami aquifer, whereas a positive sign suggests leakage is downward into the sandstone aquifer. Areas where relatively large amounts of 

deep leakage occur are indicated by large positive or 

negative head-difference values. For example, Bonita 

Springs may have the largest amount of deep leakage 

occurring into the lower Tamiami aquifer because this 

area has the largest negative head-difference values 

(about -2.0 to -0.5 m). The assumption is that the 

upper Hawthorn confining unit is continuous with a 

constant vertical hydraulic conductivity. 

Submarine ground-water discharge to the Gulf 

of Mexico, QS, occurs as ground water flowing toward 

the Gulf of Mexico moves upward and over more 

saline ground water. Fresher ground water discharges 

to the bottom of the Gulf of Mexico or Estero Bay. 

Estimates of submarine ground-water discharge are 

difficult to find in the study area. However, a recent 

study of submarine ground-water discharge from the 

Biscayne aquifer to Biscayne Bay in coastal southeastern Florida was completed by Langevin (2001). The 

average rate of submarine ground-water discharge to 

Biscayne Bay over a 10-year period was 200,000 m3

/d 

along 100 km of coastline. The Biscayne aquifer is 

more permeable and receives more recharge than the 

water-table aquifer near Bonita Springs, which suggests submarine ground-water discharge near this area 

would be less than that in Biscayne Bay.

Ground-water pumpage, QP, from the watertable and lower Tamiami aquifers in the study area 

(fig. 11) is monitored by the SFWMD. According to 

R.L. Marella (U.S. Geological Survey, written commun., 1999), ground-water withdrawals by municipal 

supply facilities in Lee and Collier Counties nearly 

doubled from 1985 to 1998 (fig. 13). Withdrawals generally are greater during the dry season (October to 

May) when rainfall is scarce than during the wet season (June to September) when rainfall is abundant 

(fig. 14). A water-use survey suggests a large volume 

of unmonitored ground-water pumpage from the lower 

Tamiami aquifer occurred for irrigation and agricultural uses (P.A. Telis, U.S. Geological Survey, written 

communication, 2000). This survey, however, was 

restricted to a small sample size with an even smaller 

number of responses. Unmonitored ground-water 

pumpage could exceed monitored ground-water 

pumpage in the lower Tamiami aquifer because the 

deepest areas of potentiometric drawdown exist at 

locations where no monitored ground-water pumpage 

is occurring (K.J. Halford, U.S. Geological Survey, 

written communication, 1999). 

Streamflow discharge, Q, is composed of surface runoff, Qro, and baseflow, QB (fig. 11). In 1996, 

the average annual streamflow (including runoff and 

baseflow) at four gaging stations was about 2 m3

/s 

(fig. 1, Spring Creek, Imperial River, and northern and 

southern branches of Estero River). The area of the 

Imperial River basin is about 2.2 x 108

 m2 (Johnson 

Engineering, Inc., and others, 1999). By dividing the 

average annual streamflow by the area of the Imperial 

River basin, the average rate of streamflow in the basin 

equates to about 29 cm/yr in 1996. The rainfall rate in 

the Imperial River basin was about 1.3 m/yr in 1996, 

as estimated from a rainfall monitoring station near 

Bonita Springs. Land use in the Imperial River basin is 

predominantly nonurban where runoff coefficients 

averaged from Bennett (1992) are about 0.1. Using 

this runoff coefficient to assume a 13-cm/yr average 

runoff rate from the 1.3-m/yr rainfall rate, the average 

rate of baseflow, Bf

, in the basin equates to about 

16 cm/yr. Higher (wet season) water levels from the 

water-table aquifer can force more baseflow to surface-water features, creating rates greater than 

16 cm/yr. Conversely, lower (dry season) water levels 

can induce baseflow rates less than 16 cm/yr.
Hydrogeology of Southwestern Florida 17

Table 1. Average 1996 water levels computed from selected monitoring wells in the lower Tamiami and sandstone aquifers

[USGS, U.S. Geological Survey; Aquifer: SS, sandstone; LT, lower Tamiami; QWDATA, Water-Quality Database; ADAPS, Automated Data Processing 

System]

Site identification Latitude Longitude

Average 

1996 head 

(meters)

Aquifer USGS data 

source

Site 

identificationLatitude Longitude

Average 

1996 head 

(meters)

Aquifer USGS data 

source

C-130 260903 814803 0.76 LT QWDATA C-1083 261856 814719 0.77 LT ADAPS

C-304 261636 813612 2.19 LT QWDATA L-738 262023 814640 .40 LT ADAPS

C-363 262556 812424 7.89 LT QWDATA L-1625 263330 813942 5.17 LT QWDATA

C-391 261124 814730 1.24 LT ADAPS L-1691 262043 814549 .80 LT QWDATA

C-458 261402 814613 1.52 LT QWDATA L-5723 262103 814643 -.03 LT QWDATA

C-460 261406 814654 1.33 LT QWDATA L-5725 261947 814902 .47 LT QWDATA

C-460 261408 814706 1.78 LT ADAPS L-5745 261925 814536 .52 LT QWDATA

C-462 262725 812606 8.73 LT QWDATA L-5747 262259 814716 -.20 LT QWDATA

C-472A 260926 814751 1.05 LT QWDATA C-298 262508 812351 6.25 SS QWDATA

C-474A 261115 814822 .47 LT QWDATA C-303 261622 814122 1.71 SS QWDATA

C-490 261244 814802 1.25 LT QWDATA C-687 262555 812837 6.66 SS QWDATA

C-492 262228 813619 5.09 LT ADAPS C-688 261803 813547 3.30 SS QWDATA

C-516 261157 814757 1.54 LT QWDATA C-689 261741 812353 3.52 SS QWDATA

C-525 261003 814836 .83 LT QWDATA C-989 261734 812854 2.61 SS QWDATA

C-526 261019 814840 .80 LT QWDATA C-1077 262823 812131 7.88 SS QWDATA

C-528 261201 814829 .68 LT QWDATA L-727 263851 813653 4.10 SS QWDATA

C-600 260550 814418 .91 LT QWDATA L-741 262553 814585 2.61 SS QWDATA

C-951 261349 813513 1.54 LT ADAPS L-1853 262707 814353 2.30 SS QWDATA

C-956 261344 813847 1.24 LT QWDATA L-1907 264309 814059 2.71 SS QWDATA

C-973 260844 813241 1.72 LT QWDATA L-1963 263345 813616 4.47 SS QWDATA

C-975 260305 813913 2.08 LT QWDATA L-1965 263354 813357 4.83 SS QWDATA

C-977 260916 813858 1.92 LT QWDATA L-1974 263719 814849 4.91 SS QWDATA

C-979 262122 813554 4.46 LT QWDATA L-1975 264400 814246 4.83 SS QWDATA

C-982 262159 812833 2.77 LT QWDATA L-1977 264321 813656 2.80 SS QWDATA

C-985 261734 812854 3.02 LT QWDATA L-2186 263345 813616 4.99 SS QWDATA

C-988 261447 812849 3.73 LT ADAPS L-2187 263951 813553 4.15 SS QWDATA

C-998 261621 814501 .71 LT QWDATA L-2192 262700 813824 5.08 SS QWDATA

C-1003 261437 814802 1.78 LT QWDATA L-2194 262022 814321 1.93 SS ADAPS

C-1004 261622 814644 .92 LT ADAPS L-2200 264330 813403 2.91 SS QWDATA

C-1058 261538 814611 .22 LT QWDATA L-2215 263128 813515 5.63 SS QWDATA

C-1064 260138 813758 .58 LT QWDATA L-2216 264609 814540 5.46 SS QWDATA

C-1066 255638 812813 -.04 LT QWDATA L-5648 263250 814743 4.61 SS QWDATA

C-1068 260315 813230 1.02 LT QWDATA L-5649 262935 814957 3.73 SS QWDATA

C-1070 260814 812142 3.85 LT QWDATA L-5664 262515 81933 2.81 SS QWDATA

C-1073 261741 812353 4.22 LT QWDATA L-5666 262514 814328 1.63 SS QWDATA

C-1074 262520 811620 7.16 LT QWDATA L-5668 262514 814717 2.57 SS QWDATA

C-1076 262823 812131 8.40 LT QWDATA L-5672 262332 813831 4.25 SS QWDATA

L-5673 262332 813831 1.90 SS QWDATA
18 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

LEE COUNTY

CHARLOTTE COUNTY

COLLIER COUNTY

HENDRY COUNTY

HENDRY COUNTY

GLADES COUNTY

GULF OF MEXICO

0

0

15 MILES

15 KILOMETERS

5 10

5 10

846

41

41

41

82

884

75

75

Caloosahatchee River

82°15 ́ 82°00 ́

26°45 ́

26°30 ́

26°15 ́

26°00 ́

81°45 ́ 81°30 ́ 81°15 ́

-2.0 to -1.6

-1.5 to -1.1

-1.0 to -0.5

-0.4 to 0.0

0.1 to 0.5

0.6 to 1.0

1.1 to 1.6

1.7 to 2.1

2.1 to 2.6

HEAD DIFFERENCE, IN METERS

EXPLANATION

Road

Canal or river

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

Implies leakage is

upward from the

sandstone aquifer into

the lower Tamiami

aquifer

Implies leakage is

downward from the

lower Tamiami aquifer

into the sandstone

aquifer

Cape

Coral

Fort

Myers

Lehigh

Acres

Naples

Bonita

Springs

Sanibel Island

Pine Island

Tamiami Trail

Figure 12. Average annual head difference between the lower Tamiami and sandstone aquifers in 1996.
Hydrogeology of Southwestern Florida 19

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

YEAR

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

AVERAGE ANNUAL GROUND-WATER

WITHDRAWALS, IN CUBIC

METERS PER DAY

AVERAGE MONTHLY GROUND-WATER

WITHDRAWALS, IN CUBIC

METERS PER DAY

0

50,000

100,000

150,000

200,000

250,000

300,000

JAN FEB MAR APR MAY JUNE AUG JULY SEP OCT NOV DEC

1995

Figure 13. Average annual ground-water withdrawals from municipal supply facilities in Lee and Collier 

Counties, 1985-98. From R.L Marella (U.S. Geological Survey, written commun. 1999).

Figure 14. Average monthly ground-water withdrawals from municipal supply facilities in Lee and Collier 

Counties, 1995.
20 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Water Quality and Geochemistry

Water-quality and geochemical data collected 

from the lower Tamiami aquifer near Bonita Springs 

were used to examine historical trends in salinity and 

to identify potential sources of saltwater. Numerous 

water samples were collected and analyzed for chloride concentration and strontium isotopes (Schmerge, 

2001). Chloride concentration is a common surrogate 

for salinity, and strontium isotopes have proven to be a 

useful tracer for determining ground-water movement 

and the origin of salinity (Clark and Fritz, 1997). 

Further explanation of methods and data analysis of 

strontium isotopes can be found in Schmerge (2001). 

Historical trends in salinity were examined for 

nine USGS monitoring wells open to the lower Tamiami aquifer near Bonita Springs; chloride concentrations ranged from about 10 to 1,300 mg/L (fig. 15). 

Monitoring well L-738 is the only well examined with 

a record that pre-dates 1985. Chloride concentrations 

in well L-738 have increased steadily from about 100 

mg/L in 1968 to about 350 mg/L in 1998. A statistical 

analysis conducted by Prinos and others (2002) 

detected a significant upward trend in chloride concentration at monitoring well L-738. Chloride concentrations in this well have increased at a rate of about 6.6 

mg/L annually from 1974 through 1998 (Prinos and 

others, 2002). Several possible explanations for this 

increasing chloride concentration are presented later in 

the discussion of ground-water flow and mechanisms 

of saltwater intrusion. In the other eight wells, chloride 

concentrations remained relatively stable or decreased 

over time.

Salinity also was examined historically by comparing areal maps of chloride concentration in the 

lower Tamiami aquifer. In a study by Knapp and others 

(1986), lines of equal chloride concentration reached a 

maximum value of 500 mg/L near Bonita Springs 

(fig. 16A). In a study by Schmerge (2001), lines of 

equal chloride concentration reached a maximum 

value of 1,000 mg/L near Bonita Springs (fig. 16B). 

Additionally, the 300-mg/L line of equal chloride concentration extended farther inland in the latter study. 

This comparison suggests saltwater occurs farther 

eastward than in 1986 in the lower Tamiami aquifer 

near Bonita Springs. Differences in the locations of 

lines of equal chloride concentrations, however, could 

be explained by differences in the amount and location 

of data available for contouring in Knapp and others 

(1986) and Schmerge (2001). With this in mind, the 

historical comparison of lines of equal chloride concentration does not conclusively determine if saltwater 

has moved since 1986. 

Water samples were collected from the lower 

Tamiami aquifer near Bonita Springs for strontium 

isotope analyses (Schmerge, 2001). Strontium isotope 

data can be used in conjunction with the hydrogeologic framework to provide evidence of the source of 

strontium in ground water (Sacks and Tihansky, 1996). 

Results suggest that the source of strontium in ground 

water sampled in the lower Tamiami aquifer near 

Bonita Springs is the underlying Floridan aquifer 

system (Schmerge, 2001). 

GROUND-WATER FLOW AND 

MECHANISMS OF SALTWATER 

INTRUSION

Ground-water flow patterns and potential mechanisms of saltwater intrusion in the study area are 

described in this section. Lateral ground-water flow in 

the lower Tamiami aquifer occurs in a southwesterly 

direction and becomes radially convergent near Bonita 

Springs. Vertical ground-water flow can be inferred 

from a map of the head difference between the lower 

Tamiami and sandstone aquifers (fig. 12). This map 

suggests that the greatest rates of vertical groundwater flow from the sandstone aquifer to the lower 

Tamiami aquifer are occurring beneath Bonita 

Springs, assuming that the upper Hawthorn confining 

unit is continuous and uniform in thickness and 

hydraulic conductivity. Approaching the saltwater 

interface, fresher ground water flowing toward the 

Gulf of Mexico moves upward and over more saline 

ground water that is flowing toward inland areas in 

lower parts of the lower Tamiami aquifer. This circulation process in the saltwater interface was observed in 

the Biscayne aquifer by Kohout (1964) and can only 

be inferred from the current monitoring well network 

established near Bonita Springs.

The distribution of saltwater in the lower 

Tamiami aquifer is likely not at equilibrium with 

current hydrologic conditions near Bonita Springs 

because of rapidly increasing rates of ground-water 

pumpage, rising sea level, and the lag in response time 

for saltwater to move to equilibrium when hydrologic 

conditions change. In fact, four potential mechanisms 

could move saltwater into zones of the lower Tamiami 

aquifer previously occupied by fresher waters. These 

mechanisms are: (1) lateral movement of the 
Ground-Water Flow and Mechanisms of Saltwater Intrusion 21

0

0

200

100

200

400

300

400

600

500

600

800

1,000

1,200

1,400

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

YEAR

YEAR

1968

1968

1998

1998

70 72 74 76 78 80 82 84 86 88 90 92 94 96

70 72 74 76 78 80 82 84 86 88 90 92 94 96

WELL L-5726

WELL C-1082

WELL L-738

WELL L-5724

WELL L-5722

WELL L-5723

EXPLANATION

EXPLANATION

WELL L-5727

WELL L-5725

WELL C-1083

Figure 15. Historical trends in chloride concentration for selected monitoring wells near Bonita Springs, 

1968-98. Well locations are shown in figure 1.
22 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida COLLIER COUNTY COLLIER COUNTY LEE COUNTY LEE COUNTY

846

846

41

41

Unit Absent

Unit Absent

500

300

300

1,000

100

200

200

50

100

50

100

300

GULF OF MEXICO

GULF OF MEXICO

EXPLANATION

Bonita

Springs

Bonita

Springs

81°45 ́

26°15 ́

26°15 ́

81°45 ́

AREA OF SALINE WATER (Sherwood and Klein, 1963)

AREA OF BRACKISH WATER (Sherwood and Klein, 1963)

APPROXIMATE EXTENT OF AQUIFER

LINE OF EQUAL CHLORIDE CONCENTRATION--

Dashed where approximately located. Hachures indicate

decreasing chloride concentration

In milligrams

per liter.

300

0

0

6 MILES

6 KILOMETERS

2 4

2 4

B

A

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

150

100

Figure 16. Lines of equal chloride concentration in the lower Tamiami 

aquifer near Bonita Springs over time. Map A modified from Knapp and 

others (1986), and map B modified from Schmerge (2001).

freshwater-saltwater interface inland from 

the southwestern coast of Florida; (2) 

upward leakage from deeper, saline waterbearing zones through natural upwelling and 

upconing; (3) downward leakage of saltwater 

from surface-water channels; and (4) movement of unflushed pockets of relict seawater 

in the lower Tamiami and water-table aquifers (fig. 17). 

Lateral Encroachment

Lateral encroachment of recent seawater into the lower Tamiami aquifer may be 

possible in southwestern Florida. Saline 

ground water beneath the Gulf of Mexico 

could move through the permeable rock 

comprising the lower Tamiami aquifer to 

come into equilibrium with modern natural 

and anthropogenic stresses, such as withdrawals, sea level, and drought (fig. 17). 

Some evidence indirectly suggests the occurrence of lateral encroachment in the lower 

Tamiami aquifer near Bonita Springs. For 

instance, the average annual potentiometric 

surface of the lower Tamiami aquifer in 1996 

is below sea level in some areas beneath 

Bonita Springs adjacent to the Gulf of Mexico; however, the predominance of lateral 

encroachment is uncertain.

Upward Leakage

A plausible mechanism for the movement of saline ground water into the freshwater zones of the lower Tamiami aquifer 

near Bonita Springs is upward leakage 

(fig. 17) either through natural upwelling or 

upconing. Natural upwelling is defined here 

as the upward movement of ground water 

within or into the lower Tamiami aquifer due 

to head differences caused by natural stressproducing processes. Natural upwelling can 

cause saltwater intrusion where salinity is 

higher in deeper locations of higher head. 

Upconing is defined here as the upward 

movement of saline ground water within or 

into the lower Tamiami aquifer due to head 

differences caused by anthropogenic stress. 

The primary difference between these two 

processes is the source of stress, either 

natural or anthropogenic.
Ground-Water Flow and Mechanisms of Saltwater Intrusion 23

LOWER TAMIAMI AQUIFER

UPPER HAWTHORN

CONFINING UNIT

SANDSTONE AQUIFER

TAMIAMI CONFINING BEDS

WATER-TABLE AQUIFER

BONITA SPRINGS CANAL WEST 

GULF OF MEXICO

KARST

FEATURES

PUMPING WELL

LEAKY

WELL

DOWNWARD LEAKAGE

NATURAL

UPWELLING

RELICT SEAWA

UPCONING

LATERAL ENCROACHMENT

EXPLANATION

Direction of ground-water flowSandstoneClaySandLimestoneBrackish ground water

Lateral encroachmentUpward leakage (through natural upwelling,upconing, karst features, and leaky wells)Downward leakageRelict seawater

MECHANISMS OF SALTWATER INTRUSION

Saline ground water

Figure 17. Potential mechanisms of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs.
24 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Diffuse natural upwelling of brackish ground 

water from the sandstone aquifer to the lower Tamiami 

aquifer probably is occurring in some parts of the 

study area. There is evidence of upward leakage to the 

base of the lower Tamiami aquifer in southwestern 

Collier County several kilometers north and south of 

I-75 and north of U.S. Highway 41 (fig. 12). These 

areas can be isolated as natural upwelling rather than 

upconing because they are characterized by sloughs, 

swamps, cypress domes, pine and/or oak islands, and 

mangrove estuaries where little anthropogenic stress, 

such as ground-water withdrawal, occurs. Upward 

leakage through natural upwelling is not apparent in 

the lower Tamiami aquifer beneath Bonita Springs 

because the potentiometric surface of the aquifer has 

been drawn down as a result of ground-water pumpage. This drawdown means natural upwelling probably 

is secondary to upconing.

Upconing of saltwater probably can be considered of utmost concern in the lower Tamiami aquifer 

beneath Bonita Springs. Upward leakage to the base of 

the lower Tamiami aquifer was inferred to be occurring at maximum rates beneath Bonita Springs in 1996 

(fig. 12), assuming the upper Hawthorn confining unit 

is continuous and uniform. However, Wedderburn and 

others (1982) report that the upper confining unit thins 

and is absent in southernmost Lee County near Bonita 

Springs. Thus, saltwater upconing can occur more easily in this area because of less resistance to vertical 

flow. Other evidence shows that water in this area of 

the lower Tamiami aquifer was less saline in the past 

(Prinos and others, 2002) and originated from deeper 

discharging aquifers (Schmerge, 2001). This latter evidence also suggests that upconing may be occurring in 

the lower Tamiami aquifer beneath Bonita Springs. 

As previously discussed, diffuse natural 

upwelling probably is not important in the lower Tamiami aquifer near Bonita Springs. Earlier studies had 

suggested that non-diffuse natural upwelling and 

upconing of saltwater through karst features could be 

occurring. Kohout (1979) identified the “mudhole” 

(fig. 1) discharging warm water to the Gulf of Mexico 

about 26 km northwest of Bonita Springs. Additionally, heads in the Floridan aquifer system are about 

10 m higher than heads in the lower Tamiami aquifer 

near Bonita Springs. Thus, ground water would naturally tend to flow from deeper, more saline, and more 

pressurized aquifer systems to the lower Tamiami 

aquifer along preferential karstic flow paths. Although 

karst features exist in the study area, this mechanism 

of saltwater intrusion probably is not predominant in 

the lower Tamiami aquifer near Bonita Springs based 

on historical chloride concentration trends (Prinos and 

others, 2002). Water in the lower Tamiami aquifer 

would have chloride concentrations similar to the 

Floridan aquifer system (about 1,200 to 1,600 mg/L) if 

karst features were naturally transporting large quantities of saltwater from deeper aquifers for thousands of 

years.

According to previous investigators (Sherwood 

and Klein, 1963; Burns, 1983; Schmerge, 2001), natural upwelling and upconing of saltwater through leaky 

wells could be occurring in the lower Tamiami aquifer 

near Bonita Springs. Additionally, a poorly sealed 

annulus of well L-2310 (not shown) was hypothesized 

as the source of saltwater intrusion in well L-738 

(fig. 15) by Schmerge (2001). Well L-2310 tapped the 

Upper Floridan aquifer and may have provided a pathway for upward leakage to the lower Tamiami aquifer 

near well L-738. The existence of other leaky wells (or 

potential leaky wells) is unknown; therefore, the predominance of leaky wells as a mechanism of saltwater 

intrusion in the lower Tamiami aquifer near Bonita 

Springs remains uncertain.

Downward Leakage

Downward leakage could occur as saline water 

from surface-water channels moves through the watertable aquifer and underlying Tamiami confining beds 

to come into equilibrium with modern natural and 

anthropogenic stresses. Downward leakage of saltwater from rivers and canals has been documented in the 

water-table aquifer near Naples (McCoy, 1962) and 

near boat basins in Naples (Klein, 1954). Downward 

leakage, however, seems an unlikely mechanism of 

saltwater intrusion in the lower Tamiami aquifer near 

Bonita Springs. Gaging stations on the northern and 

southern branches of the Estero River, Spring Creek, 

and the Imperial River record flow during the dry season, suggesting ground water flows from the watertable aquifer to these rivers and creeks even in times of 

drought. Additionally, flow from the water-table aquifer to the base of the Imperial River was observed during field reconnaissance in July 2000. Given these 

observations, it seems unlikely that saltwater moving 

upstream along Spring Creek and the Estero, Imperial, 

and Cocohatchee Rivers (fig. 1) would infiltrate 

through the water-table aquifer and underlying Tamiami confining beds and result in the distribution of 

saltwater mapped by Knapp and others (1986) and 

Schmerge (2001). 
Simulation of Saltwater Intrusion near Bonita Springs 25

Relict Seawater

Relict seawater from former high sea-level 

stands could move through the permeable rock comprising the lower Tamiami aquifer to come into equilibrium with modern natural and anthropogenic 

stresses (fig. 17). The presence of unflushed relict seawater has been hypothesized as a source of brackish 

ground water in the shallow surficial aquifer system in 

parts of southwestern Florida (Sherwood and Klein, 

1963; Schmerge, 2001). Evidence suggests, however, 

that relict seawater is not present in the lower Tamiami 

aquifer near Bonita Springs. For example, strontium 

isotope data indicate ground water in the area is from 

deeper discharging aquifers (Schmerge, 2001). Additionally, McCoy (1962) reports that flushing of most 

seawater in the shallow aquifers would have occurred 

since the Pleistocene based on ambient ground-water 

flow and recharge rates. According to Knapp and others (1986), the regional presence of unflushed water 

from sea inundations during Pleistocene interglacial 

stages is minimal.

SIMULATION OF SALTWATER 

INTRUSION NEAR BONITA SPRINGS

Numerical simulation is used to quantify modern and seasonal stresses, help identify mechanisms of 

saltwater intrusion, and estimate the potential extent of 

saltwater intrusion in the lower Tamiami aquifer near 

Bonita Springs. Numerical codes were selected to 

(1) solve the constant-density ground-water flow equation, (2) solve the variable-density ground-water flow 

and solute transport equations, and (3) facilitate model 

calibration and sensitivity analysis in three dimensions 

(table 2). Model discretization, assignment of aquifer 

properties, and assignment of boundary conditions 

provided the spatial and temporal framework necessary for solving the finite-difference equations. A predevelopment distribution of saltwater was simulated, 

and the model was calibrated to typical and modern 

stresses. The potential movement of saltwater from 

predevelopment distribution to dynamic equilibrium 

with calibrated typical, modern, and seasonal stresses 

was then simulated. Modern stresses are represented as 

stresses with the desired characteristics.

Table 2. Description and primary use of modeling tasks

Task Spatial discretization Temporal discretization Code1 Initial conditions Primary use

1

105 rows,

80 columns,

17 layers2

One steady-state flow stress 

period, representing predevelopment (1930) conditions. 

Transient transport time steps

SEAWAT

Freshwater equivalent 

heads equal to zero, a 

wall of saltwater at 

coast

Estimate predevelopment salinity conditions 

and establish initial head 

and salinity conditions 

for predictive simulations of saltwater 

movement

2 Same as above

Two steady-state flow stress 

periods, representing March 

and September 1996 conditions. Transient transport 

time steps

MODFLOW-88 Heads at land surface

Obtain an accurate representation of March and 

September 1996 head 

and flow conditions

3 Same as above

1,200 alternating steady-state 

flow stress periods, representing March and September 1996.3

 Transient transport 

time steps.

SEAWAT

Final predevelopment 

distribution of freshwater equivalent head 

and salinity

Help identify mechanisms of

saltwater intrusion and 

estimate the extent of 

saltwater intrusion in the 

lower Tamiami aquifer 

beneath Bonita Springs

1

SEAWAT (Guo and Langevin, 2002); MODFLOW-88 (McDonald and Harbaugh, 1988).

2

Aquifer properties as described in this report.

3

Ran for a total of 600 years.
26 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Modeling Approach

The modeling approach for this study can be 

summarized by three tasks (table 2):

1. Variable-density simulation of predevelopment 

conditions,

2. Calibration of a constant-density flow model to typical, modern and seasonal stresses, and

3. Variable-density predictive simulations of saltwater movement to equilibrium with the typical, modern, and seasonal stresses.

The rationale and limitations for each modeling task, 

and solely simulating advective solute transport while 

neglecting the effects of dispersion, were considered.

The first task of simulating a predevelopment 

distribution of saltwater was performed so a comparison could be made between the distribution of saltwater under predevelopment conditions and the 

distribution of saltwater at equilibrium with typical, 

modern, and seasonal stresses. This comparison was 

made to determine the potential occurrence of saltwater intrusion. Additionally, the predevelopment simulation provided a no-flow boundary condition at the 

saltwater interface for model calibration in task 2. 

Another reason for simulating a predevelopment distribution of saltwater was to develop initial conditions 

of freshwater equivalent head and ground-water salinity that were internally self-consistent for predictive 

simulations of saltwater movement (task 3). While 

performing variable-density flow simulations, consistent initial conditions of equivalent freshwater head 

and salinity will result in a model that responds to the 

imposed hydrologic stresses rather than to disequilibrium of the initial conditions.

The second task in the modeling approach is 

model calibration. A constant-density ground-water 

flow simulator was chosen for this task for two reasons:

1. Matching observed heads and flows in the study 

area was not highly dependent upon simulating 

saltwater movement. Chloride concentrations in 

observation wells were about 100 to 1,300 mg/L 

(fig. 15), suggesting the bulk of the saltwater transition zone remains either offshore, between the 

monitoring wells and the coastline, or within 

deeper hydrogeologic units. Thus, the difference 

between hydraulic-head observations and their 

simulated equivalents (when modeling) was more 

likely attributable to uncertainties in aquifer 

properties and boundary conditions, rather than 

saltwater or saltwater movement. 

2. Computer run times for constant-density groundwater flow simulations are much shorter than computer run times for variable-density flow simulations. Therefore, more time is available to test 

different conceptual models and parameter 

estimates while calibrating with a constant-density 

flow simulator. A limitation of this approach, however, was that the information contained in salinity 

observation could not be used to better estimate values of transport parameters, such as effective porosity and dispersivity. These parameters are important 

to solute transport, and thus saltwater movement.

The model was calibrated to typical, modern, 

and seasonal stresses with certain attributes. A year 

with “typical” stresses was selected to prevent approximations of saltwater movement from being biased 

toward wet or dry years. A year with “modern” 

stresses was selected to determine whether natural and 

anthropogenic stresses such as recharge, ground-water 

withdrawal, canal drainage, and sea level would move 

saltwater to areas previously occupied by fresher 

waters. “Seasonal” stresses were represented to test 

theories that alternating wet- and dry-season water levels move saltwater into and out of the lower Tamiami 

aquifer, respectively, but result in no net saltwater 

intrusion over a typical water year. 

 Typical, modern, and seasonal stresses were 

observed during 1996, so this year was chosen for 

model calibration. The year 1996 (which had about 

1.3 m of rainfall) was selected as “typical” after plotting average annual rainfall values at Bonita Springs 

Utility rainfall monitoring station (fig. 18). The rainfall data indicate that 1995 represents a wet year (with 

about 2.1 m of rainfall) and 1950 represents a dry year 

(with about 1.1 m of rainfall). The average of annual 

rainfall values in figure 18 is about 1.4 m. The 1996 

year also was selected because: (1) pumpage data 

obtained from the SFWMD began that year, and (2) 

numerous water-level and flow measurements were 

taken near Bonita Springs that year relative to other 

years. Perhaps the flooding of Bonita Springs during 

the heavy rains of 1995 (Johnson Engineering, Inc., 

and others, 1999) prompted increased awareness about 

the importance of data collection during 1996.

During model calibration, seasonal fluctuations 

in water levels were approximated by designing two 

steady-state flow stress periods, representing time 

periods in 1996 when water levels were high (wet 
Simulation of Saltwater Intrusion near Bonita Springs 27

season) and low (dry season) (table 2). By simulating 

periods with high and low water levels, the seasonal 

movement of the saltwater interface can be approximated. Additionally, a steady-state approach can be 

employed when the selected time periods show relatively stable water levels, suggesting the ground-water 

flow system is as close to steady state as realistically 

possible. There are two advantages to using the steadystate approach. First, a steady-state model does not 

require aquifer storage values. This eliminates the 

need to specify an additional parameter, which may be 

highly uncertain. Second, steady-state models tend to 

run faster than transient models because there are 

fewer input requirements and fewer solutions to the 

flow equation. Shorter computer run times allow more 

time for calibration and sensitivity analysis. A disadvantage of the steady-state approach is the possibility 

of developing a model that may fail to represent transient particularities important to the true nature of 

ground-water flow and saltwater movement near 

Bonita Springs.

March and September 1996 were selected to 

represent typical, modern, and seasonal stresses 

because ground-water levels in the water table and 

lower Tamiami aquifers generally were at or near seasonal lows and highs and also were relatively stable 

(fig. 19), justifying the steady-state assumption. Given 

these data and the above considerations, developing a 

model with two steady-state stress periods calibrated 

to average March and September 1996 conditions 

seemed to be a reasonable representation of typical, 

modern, and seasonal stresses.

Most ground-water models are calibrated by 

adjusting hydraulic conductivity until simulation 

results match with observed field conditions. The 

model documented here was calibrated by assuming 

hydraulic conductivity parameters were accurate, and 

estimating unmonitored ground-water pumpage and 

pan evaporation multipliers using UCODE. Unmonitored ground-water pumpage was estimated because 

unknown pumpage parameters caused the largest discrepancies between observed and simulated heads near 

the coast in the lower Tamiami aquifer beneath Bonita 

Springs. Accurately simulated heads in this location 

are most important for accurately simulating saltwater 

movement in this location, which was a primary objective of this study. Pan evaporation multipliers were 

estimated simultaneously with unmonitored pumpage 

because pan evaporation parameters were the most 

sensitive model parameters that were not correlated 

with unknown pumpage. Correlated parameter values 

cannot be simultaneously estimated by parameter estimation codes, such as UCODE. This is because coordinated linear changes in correlated parameter values 

produce the same simulated heads and flows at observation locations (Poeter and Hill, 1997).

The third task in the modeling approach was to 

simulate the movement of saltwater to equilibrium 

with the typical, modern, and seasonal stresses. This 

task was accomplished using the predevelopment distribution of saltwater and calibrated March and September 1996 steady-state flow stress periods. The 

calibrated steady-state flow stress periods were run in 

continual succession with a variable density groundwater flow simulator until the total salt mass in the 

simulation approached “dynamic equilibrium.” At 

dynamic equilibrium, the average annual total salt 

mass in the simulation is a constant value, even though 

some seasonal fluctuation in total salt mass may occur 

between stress periods.

YEAR YEAR

0

0.5

1.0

1.5

2.0

2.5

1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1993 1994 1995 1996 1997 1998 1999

TOTAL ANNUAL RAINFALL,

IN METERS

Figure 18. Historical record of annual rainfall at Bonita Springs Utility rainfall monitoring station, 1943-54 and 

1993-99.
28 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

A

B

AVERAGE MONTHLY WATER LEVELS IN

THE WATER-TABLE AQUIFER, IN

METERS RELATIVE TO NGVD 29

AVERAGE MONTHLY WATER LEVELS

IN THE LOWER TAMIAMI AQUIFER, IN

METERS RELATIVE TO NGVD 29

-1

-1

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

Jan 96

Jan 96

Feb 96

Feb 96

Mar 96

Mar 96

Apr 96

Apr 96

May 96

May 96

Jun 96

Jun 96

Jul 96

Jul 96

Aug 96

Aug 96

Sep 96

Sep 96

Oct 96

Oct 96

Nov 96Dec 96

Nov 96Dec 96

EXPLANATION

EXPLANATION

WELL L-2550

WELL L-2195

WELL L-1995

WELL L-1403

WELL L-730

WELL L-738

WELL C-1083

WELL C-1071

WELL C-1004

WELL C-997

WELL C-988

WELL C-968

WELL C-953

WELL C-951

WELL C-690

WELL C-496

WELL C-492

WELL C-460

WELL C-392

WELL C-391

WELL C-54

DATE

DATE

Figure 19. Average monthly water levels from selected monitoring wells in the (A) water-table and (B) lower 

Tamiami aquifers near Bonita Springs. Well locations are shown in figure 1. NGVD 29 is National Geodetic Vertical 

Datum of 1929.
Simulation of Saltwater Intrusion near Bonita Springs 29

Steady-state flow conditions were used for task 

3 by assigning a small value (1 x 10-8) for aquifer storage properties. This means that within each transport 

step, heads respond instantaneously (or nearly instantaneously) to changes in salinity. Thus, heads and 

flows for a particular transport step are always of 

steady state with salinity for that transport step. This 

approach is a simpler alternative than trying to determine appropriate storage values and run a fully transient model.

Advective solute transport was simulated while 

solving for the predevelopment distribution of saltwater and solving for the potential movement of saltwater 

from predevelopment distribution. Dispersive solute 

transport was not simulated. According to traditional 

advection-dispersion theory, hydrodynamic dispersion 

is the tendency for a solute to spread from the path it 

would be expected to follow according to advective 

hydraulics (Freeze and Cherry, 1979). Hydrodynamic 

dispersion results from molecular diffusion and 

mechanical dispersion. Molecular diffusion, the process by which a plume spreads due to concentration 

gradients, is small enough in magnitude to be ignored 

in ground-water flow systems with velocities as high 

as the system near Bonita Springs. Mechanical dispersion, another process by which a plume spreads, is 

caused by small-scale velocity variations.

Rather than assign dispersivity values, which 

would contain a high level of uncertainty, the numerical dispersion caused by the model was assumed to be 

similar in magnitude to the actual dispersion in the 

aquifer. Although there was no way to evaluate this 

assumption, it seemed justified by a lack of field data 

that could be used to calibrate dispersivity values and 

considerably long computer run times for simulations 

that directly represented dispersion. Neglecting dispersion may cause the potential extent of saltwater intrusion to be overestimated. This limitation is discussed 

in more detail in the model limitation section of the 

report. If field data are available, the effects of dispersion could be evaluated with future simulations of saltwater intrusion.

Code Selection

Two modular, three-dimensional, finite-difference models were used to simulate ground-water flow 

and salinity transport in this study (table 2). MODFLOW (McDonald and Harbaugh, 1988) was chosen 

as the constant-density ground-water flow simulator, 

and SEAWAT (Guo and Langevin, 2002) was chosen 

as the variable-density ground-water flow simulator. 

Constant-Density Ground-Water Flow Simulator

MODFLOW (McDonald and Harbaugh, 1988), 

widely accepted by the modeling community as a reliable tool, was chosen as the constant-density groundwater flow simulator. This program was used to solve 

the following steady-state ground-water flow equation:

, (2)

where Kxx, Kyy, and Kzz are values of hydraulic conductivity along the x-, y-, and z-coordinate axes, which are 

assumed to be parallel to the major axes of hydraulic 

conductivity [LT-1]; h is the potentiometric head [L], 

and W is a volumetric flux per unit volume and represents sources and/or sinks of water [T-1].

Variable-Density Ground-Water Flow Simulator

SEAWAT (Guo and Langevin, 2002), capable of 

solving for variable-density ground-water flow patterns in three dimensions, was chosen as the variabledensity ground-water flow simulator. The SEAWAT 

program explicitly or implicitly couples a variabledensity form of the ground-water flow equation solved 

with MODFLOW-88 (McDonald and Harbaugh, 

1988) to the solute transport equation solved with 

MT3Dms (Zheng and Wang, 1998). Because 

MT3Dms is used, solute concentrations or salinities 

are approximated with transient, three-dimensional, 

variable-density, ground-water flow patterns. Several 

methods are available to solve the advection and dispersion terms of the transport equation using MT3Dms 

(Zheng and Wang, 1998). Thus, the user can experiment to find a solution with acceptable levels of 

numerical dispersion and/or artificial oscillation while 

attempting to maintain a reasonable computer run 

time. 

SEAWAT reformulates the ground-water flow 

equation to solve for freshwater equivalent head as 

follows:

∂x

∂ Kxx∂x∂h

⎝ ⎠ ⎛ ⎞

∂y

∂ Kyy∂y∂h

⎝ ⎠ ⎛ ⎞

∂z

∂ Kzz∂z∂h

⎝ ⎠ + + ⎛ ⎞ – W= 0
30 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

, (3)

where x, y, and z are the principal directions of permeability,

ρ is fluid density [M/L3

],

Kf is the freshwater hydraulic conductivity [L/T],

hf

 is the freshwater equivalent head [L],

ρf is the density of freshwater [M/L3

],

Z is the vertical direction or elevation of the center of the model cell [L],

Sf

 is the freshwater equivalent storage coefficient 

[L-1],

t is time [T],

θ is effective porosity [dimensionless],

C is the solute concentration [M/L3

],

ρ is the density of water from the source or sink 

[M/L3

], and 

qs

 is the volumetric flux of water representing 

sources and sinks per unit 

volume of aquifer [T-1].

The reformulated ground-water flow equation is coupled to the following solute-transport equation:

, (4)

where 

D is the hydrodynamic dispersion coefficient 

[L2

/T], 

 is the spatial gradient operator,

 is the fluid velocity vector [L/T], 

Cs

 is the solute concentration of the source or sink 

[M/L3

], and 

Rk is the rate of solute production or decay in 

reaction k of n different reactions [M/L3

T]. 

Equations 3 and 4 are sequentially solved by 

first solving the variable-density ground-water flow 

equation and then solving the solute-transport equation. After new concentrations are calculated, a linear 

equation of state is used to relate solute concentrations 

to fluid density. The linear equation of state is:

, (5)

where E is an empirical, dimensionless constant with 

an approximate value of 0.7143 for salts commonly 

found in seawater.

Spatial Discretization and Assignment of 

Aquifer Properties

Finite-difference methods require the discretization of space into a grid consisting of rows, columns, 

and layers of model cells. Additionally, ground-water 

flow is proportional to the horizontal and vertical permeability properties of the aquifer matrix, and advective and dispersive solute transport is dependent upon 

the effective porosity of the aquifer matrix. Thus, in a 

three-dimensional, finite-difference SEAWAT model 

using the robust layer type 3 option (LAYCON 3), 

information describing the grid discretization, horizontal hydraulic conductivity, vertical conductance 

(VCONT) (McDonald and Harbaugh, 1988), and 

effective porosity must be written to model input files. 

The aquifer property information must be assigned to 

model cells in a way that reflects the bulk occurrence 

of these properties within the entire volume of the 

model cell.

The study area was spatially discretized into a 

grid with 105 rows, 80 columns, and 17 layers of 

square cells. Seventeen layers were used to obtain sufficient vertical resolution of the saltwater interface. 

Each model cell is 600 by 600 m and 5 m thick, except 

for layer 1 where the cell top varies to match land surface, and the cell bottom is 5 m below National Geodetic Vertical Datum (NGVD) of 1929. Uniform cell 

volumes were used, except for layer 1, to minimize 

mass-balance errors that could occur if a mixed Eulerian-Lagrangian method was needed to solve the 

advection term of the transport equation. Examples of 

such a method with MT3Dms are the method of characteristics (MOC), modified method of characteristics 

(MMOC), or hybrid method of characteristics 

(HMOC). Eulerian-Lagrangian methods do not 

guarantee local mass conservation at a particular time 

step because of the discrete nature of particle tracking 

used in these techniques (Zheng and Wang, 1998). 

∂

∂x

----- ρKfx

∂hf

∂x

------- ρ ρf –

ρf

--------------∂Z

∂x

+ ------ ⎝ ⎠ ⎛ ⎞

∂

∂y

----- ρKfy

∂hf

∂y

------- ρ ρf –

ρf

--------------∂Z

∂y

+ ------ ⎝ ⎠ ⎛ ⎞

∂

∂z

----- ρKfz

∂hf

∂z

------- ρ ρf –

ρf

--------------∂Z

∂z

+ ------ ⎝ ⎠ ⎛ ⎞

ρSf

∂hf

∂t ------- θ ∂ρ

∂C

------∂C

∂t

------ ρqs + –

=

+

+

∇( )∇ D∇C ( ) vC qs

θ • – ----Cs – Rk

k = 1

n

+ ∑ ∂C

∂t = ------

∇

v

ρ ρ = f + EC
Simulation of Saltwater Intrusion near Bonita Springs 31

Mass-balance errors can be exacerbated by nonuniform cell volumes.

Bulk values of horizontal hydraulic conductivity 

and VCONT were assigned to model cells using a 

three-dimensional conceptual permeability model, the 

configuration of the model grid, and weighted arithmetic or harmonic averaging. The three-dimensional 

conceptual permeability model was developed by 

assigning the values of horizontal and vertical hydraulic conductivity, shown in table 3, to the framework of 

the water-table aquifer, Tamiami confining beds, lower 

Tamiami aquifer, upper Hawthorn confining unit, and 

sandstone aquifer. Because hydraulic conductivity typically is considered to be log-normally distributed, permeability values were assigned (as described below) 

using the log K histograms of aquifer horizontal 

hydraulic conductivity (fig. 4) compiled from: 

(1) previous literature (Bower and others, 1990), and 

(2) reanalysis of previous aquifer performance tests 

(K.M. Edwards, U.S. Geological Survey, written 

commun., 2000).

The water-table aquifer was assigned a horizontal hydraulic conductivity of 300 m/d (table 3). The 

horizontal hydraulic conductivity was based on the 

inverse log of the midpoint of the first mode in the distribution of 244 measurements (fig. 4A). A vertical 

hydraulic conductivity of 15 m/d was assigned to the 

water-table aquifer (table 3) because reanalysis of 

aquifer performance tests in the study area by 

K.M. Edwards (U.S. Geological Survey, written commun., 2000) suggests that the anisotropy ratio of horizontal to vertical hydraulic conductivity is about 20:1 

in the water-table aquifer. 

The Tamiami confining beds and the upper 

Hawthorn confining unit were assigned horizontal 

hydraulic conductivities of 0.04 and 0.03 m/d, respectively, and vertical hydraulic conductivities of 0.004 

and 0.003 m/d, respectively (table 3). The same vertical hydraulic conductivities (0.004 and 0.003 m/d) 

were measured by Montgomery (1988). Little published information was available to constrain values of 

horizontal hydraulic conductivity for both confining 

units. In modeling applications, the ratio of horizontal 

hydraulic conductivity to vertical hydraulic conductivity is often assumed to range from 1 to 1,000 (Anderson and Woessner, 1992). Thus, the horizontal 

hydraulic conductivities of the Tamiami confining 

beds and the upper Hawthorn confining unit were set 

to 0.04 and 0.03 m/d, respectively, or 10 times the 

measured vertical hydraulic conductivity found by 

Montgomery (1988). This may seem somewhat arbitrary, but the results of ground-water flow models typically are insensitive to values of horizontal hydraulic 

conductivity in confining units. Because of the low 

permeability of these units, horizontal flow within 

them is essentially negligible.

In the lower Tamiami aquifer, horizontal 

hydraulic conductivity appears to have a bimodal log 

K distribution (fig. 4B). Results from aquifer performance tests compiled for this study (figs. 4B and 8) 

and for previous research (Meeder, 1979) suggest that 

permeabilities in the lower Tamiami aquifer are much 

greater in areas where coral reef facies are present. The 

lower Tamiami aquifer was assigned a horizontal 

hydraulic conductivity value of 130 m/d in areas where 

the coral reef facies are known to exist (table 3). This 

value was the inverse log of the arithmetic average log 

K value computed from eight aquifer performance 

tests completed in coral reef facies (fig. 8). Elsewhere, 

the lower Tamiami aquifer was assigned a horizontal 

hydraulic conductivity of 70 m/d (table 3) − the inverse 

log of the arithmetic average log K value computed 

from 136 measurements (fig. 4B). A vertical hydraulic 

conductivity value of 6.5 m/d was assigned in areas of 

the lower Tamiami aquifer where coral reef facies are 

present and 3.5 m/d elsewhere because reanalysis of 

aquifer performance tests suggests the anisotropy ratio 

of horizontal to vertical hydraulic conductivity is about 

20:1 in the aquifer.

Table 3. Hydraulic conductivities used in the conceptual 

permeability model 

Hydrogeologic unit

Hydraulic conductivity

(meters per day)

Horizontal Vertical 

Water-table aquifer 300 15

Tamiami confining beds .04 .004

Lower Tamiami aquifer 70 3.5

Lower Tamiami aquifer, coral reef 

facies

 130 6.5

Upper Hawthorn confining unit .03 .003

Sandstone aquifer 20 2
32 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

The sandstone aquifer was assigned a horizontal 

hydraulic conductivity of 20 m/d and a vertical 

hydraulic conductivity of 2 m/d (table 3). The horizontal hydraulic conductivity was based on the inverse log 

of the arithmetic average log K value computed from 

the distribution of 25 measurements (fig. 4C). Little 

published information was available to constrain values of vertical hydraulic conductivity for the sandstone 

aquifer. The vertical hydraulic conductivity was based 

on the 1 to 1,000 range of vertical to horizontal conductivity ratios. Thus, the vertical hydraulic conductivity of the sandstone aquifer was set to 2 m/d, or onetenth the horizontal hydraulic conductivity. Again, this 

may seem somewhat arbitrary, but model results are 

expected to be insensitive to the vertical hydraulic conductivity of the sandstone aquifer. Because the focus 

of this study is the lower Tamiami aquifer, flow into or 

out of the aquifer will be controlled more by the vertical hydraulic conductivity of the underlying upper 

Hawthorn confining unit than the vertical hydraulic 

conductivity of the sandstone aquifer.

Because the hydrogeologic units do not coincide 

with model layers, bulk values of horizontal hydraulic 

conductivity (fig. 20) were computed as the weighted 

arithmetic mean of the hydraulic conductivities (table 

3) that lie within the model cell (fig. 21). The weighted 

arithmetic mean is expressed as:

, (6)

where Kxy i,j,k is the bulk horizontal hydraulic conductivity of the model cell i,j,k [L/T], u is a counter ranging from 1 to n, in which n is the number of 

hydrogeologic units within the model cell [dimensionless], Wu is weight representing the fraction of the 

model cell’s thickness occupied by the hydrogeologic 

unit (the sum of the weights for each model cell must 

equal 1 [dimensionless]), and Kxyu is the horizontal 

hydraulic conductivity of the hydrogeologic unit 

[L/T]. 

INACTIVE CELLS

EXPLANATION

HORIZONTAL HYDRAULIC

CONDUCTIVITY, IN

METERS PER DAY

Estero Bay

BONITA SPRINGS

Cropped at

row 39

Cropped at

column 45

Column 1

Row 1

Figure 20. Distribution of bulk horizontal hydraulic conductivity in selected model cells.

Kxyijk , , WuKxyu

u = 1

n

= ∑
Simulation of Saltwater Intrusion near Bonita Springs 33

Bulk values of vertical hydraulic conductivity 

for model cells were computed as the weighted harmonic mean of hydraulic conductivities (table 3) that 

lie within the model cell (fig. 21). A three-dimensional 

diagram of bulk values of vertical hydraulic conductivity would look similar to the three-dimensional diagram of bulk values of horizontal hydraulic 

conductivity (fig. 20), except values would range 

would from 15 to 0.003 m/d. The weighted harmonic 

mean is expressed as:

, (7)

where Kzi,j,k is the bulk vertical hydraulic conductivity 

of the model cell [L/T], and Kzu is the vertical hydraulic conductivity of the hydrogeologic unit [L/T].

Except for the bottom layer (layer 17), a 

VCONT term is required input for each model cell in a 

MODFLOW or SEAWAT simulation. Bulk values of 

VCONT were computed as:

, (8)

where VCONTi,j,k, is the required VCONT term [1/T], 

and ∆bi,j,k is the thickness of the model cell [L].

Estimates of effective porosity were needed for 

variable-density ground-water flow simulations. Effective porosity, the percentage of interconnected pore 

space within geologic material, was uniformly set to 

30 percent. This percentage is based on analysis of 

exploratory cores collected from the gray limestone 

and lower Tamiami aquifers in the study area (Reese 

and Cunningham, 2000).

W1

W2

W3

HYDROGEOLOGIC UNIT 1

KXY1, KZ1

HYDROGEOLOGIC UNIT 2

KXY2, KZ2

HYDROGEOLOGIC UNIT 3

KXY3, KZ3

EXPLANATION

W

KXY

Z

X

KZ

Bulk horizontal hydraulic conductivity

Bulk vertical hydraulic conductivity

Weight representing the relative thickness

of the hydrogeologic unit

Figure 21. Approach used to compute bulk hydraulic conductivities for each model cell using weighted 

arithmetic and harmonic averaging.

Kzijk , ,

1

Wu

1

Kzu

--------

u = 1

n

∑

= ---------------------------

VCONTijk , ,

2 Kzijk , , ( ) Kzijk , , + 1 ( )

∆bijk , , Kzijk , , ( )∆bijk , , + 1 Kzijk , , + 1 + ( ) -------------------------------------------------------------------------------------------------------

=
34 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Simulation of Predevelopment Conditions

In the first modeling task, the SEAWAT program 

was used to simulate the location of the interface prior 

to development. The year 1930 was selected to represent predevelopment conditions primarily because the 

network of canals had not yet been constructed 

(Ananta Nath, Big Cypress Basin, oral commun., 

2001), and ground-water pumpage probably was negligible. A single steady-state stress period was used to 

represent conditions in 1930. The spatial discretization 

and aquifer properties previously described were used 

for the simulation. Results from the predevelopment 

model consist of three-dimensional distributions of 

head and salinity. These model results are used to help 

define a coastal boundary condition for model calibration (task 2). Results from the predevelopment simulation also are used as initial conditions for predictive 

simulations of saltwater movement (task 3).

Boundary Conditions

Boundary conditions for simulating the predevelopment distribution of saltwater include rainfall, 

evapotranspiration, rivers, lateral conditions of no 

flows, and constant heads with constant concentrations 

(pl. 1). Rainfall and evapotranspiration boundary conditions were used to allow the simulation to calculate 

the net recharge to the top of active model cells located 

inland from the coastline. Because rainfall and evapotranspiration are assigned to active cells in layer 1, 

these boundary conditions are not shaded nor labeled 

on plate 1. River boundary conditions were used to 

represent Spring Creek, and the Imperial, Cocohatchee 

and Estero Rivers. No-flow boundary conditions were 

used to represent predevelopment hydrologic divides 

that were assumed to exist along modern ground-water 

flow lines. Because each cell in layers 16 and 17 is a 

no-flow boundary condition, only layers 1 to 15 are 

plotted on plate 1. Layers 16 and 17 were kept in the 

model, however, in case they are needed to represent 

deeper hydrogeologic units in future investigations. 

Constant-head and constant-concentration boundary 

conditions were used to represent the Gulf of Mexico. 

The salinity of inflow from each source boundary condition was set to zero, with exception to the boundary 

conditions representing the Gulf of Mexico, which 

were assigned a salinity of seawater.

Recharge

Limited data were available that describe 

recharge in 1930; therefore, recharge was simulated 

using the approach described by Motz (1996) and 

Stewart and Langevin (1999). Using this approach, 

both rainfall and evapotranspiration are included in the 

model, with the maximum potential ground-water 

evapotranspiration rate set equal to the rainfall rate. 

This creates an aquifer system where water levels are 

controlled by land-surface topography (Motz, 1996). 

Using this method to simulate recharge is advantageous because spatial distributions of net recharge are 

generated rather than discrete zones. Disadvantages 

include the potential for additional uncertainty in 

model simulations due to the lack of data to support 

estimates of rainfall, maximum potential evapotranspiration rates, extinction depths, and the evapotranspiration surface. 

Rainfall was entered using the Recharge package (McDonald and Harbaugh, 1988). The Recharge 

package requires input of an areal flux [L/T] to the top 

grid layer, a specified vertical distribution of layers, or 

the highest active cell in a vertical column of layers. 

The areal recharge flux was added to the highest active 

cell in a vertical column of layers. Average annual 

rainfall rates were computed from historical rainfall 

recorded at the Bonita Springs monitoring station from 

1943 to 1999 (fig. 18). Years with no average annual 

rainfall rate indicate missing or insufficient data 

(1955-92). No trends of annual average rainfall rates 

are evident, thus an extrapolation relation to compute 

the average annual rainfall rate in 1930 was not necessary. The average of annual average rainfall rates was 

about 1.3 m/yr and was written as the areal flux in the 

Recharge package.

Evapotranspiration was entered using the 

Evapotranspiration package (McDonald and Harbaugh, 1988) where evapotranspiration is approximated as a linearly varying rate that is greatest at the 

evapotranspiration surface and decreases to zero at the 

extinction depth. As required by the coupled aquifer 

approach, the maximum evapotranspiration rate of the 

evapotranspiration surface was set equal to the rainfall 

rate. The evapotranspiration surface was set to land 

surface, and extinction depths were set to 5 m below 

land surface.

Extinction depths are uncertain in the study 

area. Extinction depths at about 0.3 to 2.1 m were used 

in previous modeling studies of southwestern Florida 

(Bower and others, 1990; Bennett, 1992). These 

depths were related to land use and based upon estimated root depths for various types of vegetation. In 

addition to vegetation root depth, evapotranspiration is 
Simulation of Saltwater Intrusion near Bonita Springs 35

dependent upon other processes and properties, such 

as climate and soil type. For example, fine-grained 

soils will have deeper extinction depths than coarsegrained soils because more energy is required to lift 

ground water through coarse-grained soils.

A network of evapotranspiration stations maintained in southern Florida by the USGS suggests 

extinction depths of at least 1 m over many types of 

land cover (S.L. Kinnaman, U.S. Geological Survey, 

written commun., 2001). Tibbals (1990) estimated 

evapotranspiration rates of about 76 cm/yr in areas 

where the water table was about 4 m below land surface in east-central Florida. Merritt (1996) used extinction depths up to 6 m below land surface in a coastal 

ridge area of southeastern Florida. Setting extinction 

depths to 5 m below land surface seemed to be a reasonable compromise, given the current understanding 

of evapotranspiration in Florida.

Rivers

Predevelopment river boundary conditions (pl. 

1) were entered using the River package (McDonald 

and Harbaugh, 1988). The conceptual model for the 

River package is based on the vertical leakage of water 

across river bottom sediments. The cross section of the 

river is conceptualized in the model code as rectangular with impermeable vertical sides. Leakage into or 

out of the model cell is dependent on the stage in the 

river (hriv), the value of head in the model cell (hi,j,k), 

and a conductance term. The conductance term of the 

river is mathematically defined as:

, (9)

where CONDriv is the conductance of the river bed 

[L2

/T], 

L is the length of the river segment in the model 

cell [L], 

w is the bottom width of the river [L], 

Kseds is the vertical hydraulic conductivity of the 

river bottom sediments [L/T], and 

bseds is the thickness of the river bottom sediments 

[L].

The elevation of the base of the river bottom 

sediments is specified as RBOT, an input parameter for 

the River package. When hi,j,k is greater than RBOT, 

leakage from the aquifer to the river, Qriv, is calculated 

as:

. (10)

When hi,j,k is less than RBOT, the leakage rate from the 

river to the aquifer is calculated as:

. (11)

The Estero River, Imperial River, Cocohatchee 

River, and Spring Creek (fig. 1) were simulated with 

the River package. These surface-water features were 

digitized from a USGS 1:100,000-scale topographicbathymetric map. For predevelopment conditions, an 

attempt was made to digitize only parts of the river and 

creek channels that appeared to be unaltered. Input for 

the River package includes the layer, row, column, hriv, 

CONDriv, and RBOT. The layer, row, and column of 

river cells were determined by overlying the digitized 

river features with the model grid. The river stage, hriv, 

was estimated at 0.5 m below the land-surface elevation of the model cell because this value seems reasonable, and no quantitative data describing river stages in 

1930 exist. For each river cell, CONDriv was computed 

by multiplying the area of the river or creek channel 

within the model cell by Kseds . The value of Kseds is 

assumed to be 15 m/d, which is the vertical hydraulic 

conductivity used for the water-table aquifer (table 3); 

the value of bseds was set to 1 m for lack of a better 

value, and the value of RBOT was estimated at 2 m less 

than hriv.

There may be some reason for concern about the 

lack of predevelopment data for assigning river parameter values. It is likely, however, that these parameters 

have little effect on the goal of this simulation, which 

is to simulate a predevelopment distribution of saltwater. River parameters control a relatively small component of the water budget in this simulation; namely, 

flow between the aquifer and surface-water features. 

Parameters that control small components of the water 

budget typically are less sensitive than parameters that 

control large components of the water budget.

No Flow

Ground-water divides observed in the watertable and lower Tamiami aquifers during March and 

September 1996 (fig. 22) were represented as no-flow 

boundary conditions in the predevelopment simulation 

(pl. 1). Representing a ground-water divide as a noflow boundary is conceptually consistent with the 

actual behavior of the divide; ground water on each 

side of the divide moves away, and no flow crosses the 

divide (Reilly, 2001).

CONDriv

LwKseds

bseds

= --------------------

Qriv CONDriv hriv hijk , , = ( ) –

Qriv CONDriv h = ( ) riv – RBOT
36 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Bonita

Springs

Bonita

Springs

Bonita

Springs

Bonita

Springs

HENDRY COUNTY

HENDRY COUNTY

HENDRY COUNTY

HENDRY COUNTY

COLLIER COUNTY

COLLIER COUNTY

COLLIER COUNTY

COLLIER COUNTY

LEE COUNTY

LEE COUNTY

LEE COUNTY

LEE COUNTY

LOWER TAMIAMI AQUIFER

MARCH 1996

EXPLANATION

LINE OF EQUAL WATER LEVEL--Shown

in meters relative to NGVD 29. Dashed where

approximately located. Interval is 2 meters

WELL LOCATION AND VALUE-- Average

1996 water level shown in meters

GROUND-WATER DIVIDE

LOWER TAMIAMI AQUIFER

SEPTEMBER 1996

2

2

2

2

2

2

2

2

4

4

6

0

4

4

8

4

6

6

6

0 15 MILES 5 10

0 15 KILOMETERS 5 10

0 15 MILES 5 10

0 15 KILOMETERS 5 10

0 15 MILES 5 10

0 15 KILOMETERS 5 10

0 15 MILES 5 10

0 15 KILOMETERS 5 10

0.9

1.0

1.1

1.4

2.0 1.9

5.2

0.0 0.5

0.8

1.1

1.5

-1.3

-1.3

0.0

0.0

0.5 0.6

0.8

0.8

1.2

1.2

1.3

1.4

1.6

1.7

1.7

2.0

3.1

3.5

3.6

4.0

8.0

5.0

0.1

0.4

0.6

0.6

1.2

1.3

1.5

1.9

1.9

1.9

1.9

2.1

2.6

2.9

3.0

3.4 3.3

3.5

3.9

4.0

4.1

4.8

4.8

5.3

6.3

6.6

6.7 7.0

7.2

7.9

8.3

1.5 1.7

2.1

2.4

3.6

3.7

5.0

7.5

8.2

WATER-TABLE AQUIFER

MARCH 1996

WATER-TABLE AQUIFER

SEPTEMBER 1996

82°00 ́

82°00 ́

82°00 ́

82°00 ́

81°45 ́

81°45 ́

81°45 ́

81°45 ́

81°30 ́

81°30 ́

81°30 ́

81°30 ́

26°30 ́

26°30 ́

26°30 ́

26°30 ́

26°15 ́

26°00 ́

26°00 ́

26°00 ́

26°00 ́

26°15 ́

26°15 ́

26°15 ́

3.6

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

GULF OF MEXICO

GULF OF MEXICO

GULF OF MEXICO

GULF OF MEXICO

Figure 22. Average monthly water levels in the water-table and lower Tamiami aquifers during March and 

September 1996. NGVD 29 is National Geodetic Vertical Datum of 1929.
Simulation of Saltwater Intrusion near Bonita Springs 37

Ground-water divides observed in the watertable and lower Tamiami aquifers during March and 

September 1996 may or may not have been in the same 

location in 1930. It seems reasonable to expect that the 

northern and southern ground-water divides remain 

relatively stable. Recharge on the Florida Peninsula 

creates a mound of ground water in aquifers above sea 

level, resulting in flow from inland areas toward the 

coast. Additionally, contemporary ground-water withdrawal near these northern and southern divides is less 

prevalent, resulting in less opportunity for stress to 

move the location of these divides. The location of the 

eastern ground-water divide during 1930, however, is 

more uncertain; in fact, the divide may never have 

existed. This boundary condition probably is far 

enough away from the saltwater transition zone in the 

lower Tamiami aquifer to have little consequence on 

model results. Regardless, a better method for approximating the location of lateral predevelopment boundary conditions would reduce uncertainty in model 

results.

Constant Heads and Concentrations

Constant-head boundary conditions were 

entered into SEAWAT using the IBOUND and Shead 

arrays of the Basic package (McDonald and Harbaugh, 

1988). An integer value less than zero in the IBOUND 

array of the Basic package indicates that the model cell 

has a constant-head throughout the simulation. The 

constant-head value will be equal to a real number 

entered in the same location of the Shead array of the 

Basic package. Constant-concentration boundary conditions were entered using the ICBUND array in the 

Basic Transport package and the Source Sink Mixing 

package (Zheng and Wang, 1998). An integer value 

less than zero in the ICBUND array of the Basic 

Transport package indicates the model cell has a constant concentration equal to a value specified for that 

model cell in the Source Sink Mixing package.

Constant-concentration boundary conditions 

were used for layer 1 and along the western boundary 

of the model in layers 2 to 15 to represent the Gulf of 

Mexico (pl. 1). The boundary conditions were 

assigned a constant salt concentration equal to 

35 kg/m3

 (the salinity of seawater).

Constant-head boundary conditions were used 

in layer 1 and along the western boundary of the model 

in layers 2 to 15 to represent the Gulf of Mexico 

(pl. 1). Constant-head values representing the Gulf of 

Mexico in 1930 were assigned by: (1) computing the 

average value of sea level in 1996 using daily sea level 

measurements recorded at National Oceanic and 

Atmospheric Administration (NOAA) station 8725110 

(not shown), (2) extrapolating sea level backward to 

1930 using the average 1996 value and a 2-mm/yr rate 

of sea-level rise (National Research Council, 1987), 

and (3) converting the extrapolated 1930 sea level 

value to a hydrostatic freshwater equivalent sea level 

for input to the SEAWAT simulator. On average, sea 

level in 1996 was about 0.2 m above NGVD 1929. By 

extrapolating, sea level in 1930 was calculated to be 

about 0.07 m above NVGD 1929. Hydrostatic freshwater equivalent sea level (hf

) was computed using the 

following equation:

. (12)

Z was set to the elevation of the center of the 

model cell. Fluid density (ρ) was set to 1,025 kg/m3

(density of saltwater). Extrapolated 1930 sea level (hs

) 

was computed as 0.07 m. Using the equation and data 

above, the freshwater equivalent sea level in 1930 was 

0.12 m above NVGD 1929 for layer 1 and proportionately greater than 0.12 m at the western boundary of 

the model in layers 2 to 15 because the elevation of the 

center of these model cells, Z, is farther below NVGD 

1929.

Simulation Results

The predevelopment simulation was run until 

heads and salinity reached steady state. An evaluation 

of simulated predevelopment heads (not shown) suggested that the model provides a reasonable representation of coastal ground-water flow. An estimate of the 

predevelopment distribution of saltwater is obtained by 

evaluating the solute concentrations simulated by the 

model.

A three-dimensional view of the predevelopment distribution of saltwater was prepared using 

Modelviewer, a visualization and animation program 

(Hsieh and Winston, 2002) (fig. 23). Although this 

view provides a more complete perspective of the predevelopment distribution of saltwater, it is difficult to 

visualize how ground-water salinity relates with other 

prominent features in the study area. Thus, a twodimensional map showing the intersection of the predevelopment saltwater interface with the base of the 

lower Tamiami aquifer was prepared (fig. 24).

hf Z ρ

ρf

---- hs = + ( ) – Z
38 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

The predevelopment distribution of saltwater 

consists of a sharp, steep interface that roughly parallels the coast. Near Naples and central Estero Bay, the 

saltwater interface intersects with the base of the lower 

Tamiami aquifer directly beneath the coastline. In 

most other areas, the intersection is as much as 2 km 

offshore. Model results suggest the interface is directly 

beneath the coastline in locations where less ground 

water is discharging to the Gulf of Mexico. Conversely, the interface is farther from the coastline in 

locations where more ground water is discharging to 

the Gulf of Mexico. These results appear to be a reasonable estimate of salinity conditions in the watertable and lower Tamiami aquifers prior to modern 

stresses, such as ground-water development and canal 

drainage.

Model Calibration to Typical, Modern, and 

Seasonal Stresses

Model calibration was necessary to move the 

predevelopment distribution of saltwater to equilibrium with a reasonable representation of March and 

September 1996 conditions (table 2). The same spatial 

discretization and aquifer properties previously 

described were used for calibration. Two steady-state 

stress periods representing average March and 

September 1996 conditions were designed. The 

MODFLOW program was used to simulate groundwater flow.

The calibration objective of this study was to 

find the combination of model parameters or boundary 

conditions that best represent March and September 

1996 conditions. Boundary conditions were assigned 

to closely resemble the time periods of interest. The 

parameter estimation routine, UCODE (Poeter and 

Hill, 1998), was used to “best fit” simulated heads and 

flows to their observed equivalents by solving for 

parameter estimates using a nonlinear least squares 

regression. The reliability or uncertainty in parameter 

estimates also was quantified with linear 95-percent 

confidence intervals.

Boundary Conditions

Boundary conditions for model calibration 

under March and September 1996 hydrologic conditions include no flow, constant heads, rivers, general 

heads, rainfall, evapotranspiration, and wells (pl. 2). 

No-flow boundary conditions were used to represent 

hydrologic divides and the saltwater interface. Layers 

16 and 17 consist entirely of no-flow cells; therefore, 

these layers are not plotted on plate 2. Constant-head 

boundary conditions were used to represent the Gulf 

of Mexico. River boundary conditions were used to 

SALTWATER INTERFACE

BONITA SPRINGS GULF OF MEXICO

N

EXPLANATION

SALINITY OF GROUND WATER,

IN KILOGRAMS PER CUBIC METER

0 17.5 35

Figure 23. Three-dimensional view of the simulated predevelopment distribution of saltwater. All shaded 

areas represent active model cells. Blue and tan cells are equivalent in salinity; the tan shading is used to 

highlight the topography of the base of the lower Tamiami aquifer.
Simulation of Saltwater Intrusion near Bonita Springs 39

represent local rivers and the system of canals in the 

southeastern part of the study area. General-head 

boundary conditions were used to represent deep leakage of ground water to and from the base of the lower 

Tamiami aquifer. Rainfall and evapotranspiration 

boundary conditions were used to represent net 

recharge; however, these boundary conditions are not 

plotted or labeled on plate 2 for the same reasons they 

are not plotted on plate 1 as previously mentioned. 

Well boundary conditions were used to represent 

ground-water pumpage from the water-table and lower 

Tamiami aquifers.

SALTWATER INTERFACE AT THE BASE

OF THE LOWER TAMIAMI AQUIFER

Estero Bay

GULF OF MEXICO

0

0

6 MILES

6 KILOMETERS

2 4

2 4

26°15 ́

81°45 ́

Naples

LEE COUNTY

Bonita

Springs

COLLIER COUNTY

Trail

Tamiami

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17 EXPLANATION

Cocohatchee

River

Imperial

River

Spring

Creek

Alligator Alley

75

41

Figure 24. Approximate position of the simulated predevelopment saltwater interface at the base of the 

lower Tamiami aquifer.
40 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

No Flow

The Basic package (McDonald and Harbaugh, 

1988) was used to set no-flow boundary conditions 

that represent hydrologic divides and the saltwater 

interface (pl. 2). The input and mechanics of accomplishing this using the Basic package were previously 

described. No-flow boundary conditions representing 

hydrologic divides were assigned to model layers at 

the northern, eastern, and southern boundaries of the 

simulation for both steady-state stress periods. These 

boundary conditions were set using water-level contour maps of the water-table and lower Tamiami aquifers during March and September 1996 (fig. 22). 

No-flow boundary conditions representing the 

saltwater interface in layers 2 to 15 beneath the Gulf of 

Mexico were set using the predevelopment distribution 

of saltwater. All model cells west of the 50-percent 

seawater zone in the predevelopment simulation were 

designated as no-flow cells in both steady-state stress 

periods. In systems where density changes abruptly 

between a freshwater zone and a more dense “salty” 

zone, the boundary between freshwater and saltwater 

can be conceptualized as a no-flow boundary condition when movement of the boundary is assumed to be 

of negligible importance to the problem (Reilly, 2001). 

Most water-level and flow observations for calibrating 

the model to March and September 1996 conditions 

are several kilometers inland with chloride concentrations less than 1,000 mg/L. This suggests the bulk of 

the saltwater interface has not moved through the 

vicinity of these observations, and the error in simulated heads and flows will more likely be attributable 

to uncertainty in rock properties and other boundary 

conditions, such as recharge, than from the position or 

movement of saltwater or brackish water with greater 

fluid density.

Constant Heads

The Constant Head Boundary (CHD) package 

(Leake and Prudic, 1991) was used to set constanthead boundary conditions that represent the average 

elevation of sea level in the Gulf of Mexico during 

March and September 1996. The IBOUND and Shead 

arrays in the Basic package could not be used because 

the average elevation of sea level in March 1996 (the 

first steady-state stress period) is different than the 

average elevation of sea level in September 1996 (the 

second steady-state stress period). The IBOUND and 

Shead arrays of the Basic package can apply only one 

constant head value entered in the Shead array for the 

entire length of a simulation. 

The CHD package requires the input of the 

layer, row, column, starting head, and ending head values for the constant-head cell for each stress period. 

The ARC/INFO coverages of the Gulf of Mexico were 

overlain with ARC/INFO coverages of the model grid 

to identify rows and columns of model cells in layer 1 

that needed to be constant heads. The starting and ending head values for the first steady-state stress period 

representing March 1996 were set equal to the average 

elevation of sea level in the Gulf of Mexico recorded at 

Naples station 8725110 (about 0.1 m). The starting 

and ending head values for the second steady-state 

stress period representing September 1996 were set 

equal to the average elevation of sea level in the Gulf 

of Mexico recorded by NOAA station 8725110 (about 

0.3 m). By setting the starting and ending head values 

equal, a constant-head boundary condition was 

enforced with a different value for each steady-state 

stress period.

Rivers

The River package (McDonald and Harbaugh, 

1988) was used to set river boundary conditions that 

represent leakage of water to and from the groundwater flow system through surface-water channels 

(pl. 2). The input and mechanics of the River package 

(McDonald and Harbaugh, 1988) were previously 

described. For calibrating the model to March and 

September 1996 conditions, the River package 

requires the layer, row, column, hriv, CONDriv, and 

RBOT for both steady-state stress periods. The layer, 

row, and column of river cells were determined by 

overlying ARC/INFO coverages of contemporary 

surface-water features with the model grid. The 

USGS or SFWMD surface-water gaging stations, if 

available, were used to interpolate or extrapolate average March and September 1996 hriv values for both 

steady-state stress periods. For each river cell, 

CONDriv was computed using equation 9. The value of 

Kseds is assumed to be 15 m/d, which is the vertical 

hydraulic conductivity used for the water-table aquifer 

(table 3); the value of bseds was given a reasonable 

value of 1 m, and the value of RBOT was about 2 m 

less than hriv.
Simulation of Saltwater Intrusion near Bonita Springs 41

General Heads

The General-Head Boundary (GHB) package 

(McDonald and Harbaugh, 1988) was used to set general-head boundary conditions that represent the deep 

leakage of ground water to and from the base of the 

lower Tamiami aquifer and the sandstone aquifer during March and September 1996 (pl. 2). In MODFLOW-88, the GHB package is one of the most robust 

packages available for simulating a wide range of 

boundary conditions. General-head boundaries are 

head-dependent boundaries where the volumetric flux, 

QGHB, is proportional to the head difference between an 

assigned external head and the approximated head in 

the model cell. The form of Darcy's law used to characterize the flux is:

, (13)

where CONDGHB is the conductance of the generalhead boundary [L2

/T], and hGHB is the assigned external head value of the general-head boundary [L]. 

The conductance value, CONDGHB, of the general-head boundary was mathematically defined as:

, (14)

where L is the length of the model cell [L], w is the 

width of the model cell [L], KGHB is the vertical 

hydraulic conductivity of the upper Hawthorn confining unit [L/T], and bGHB is the thickness of the upper 

Hawthorn confining unit [L].

 Variables L and w of the model cells were 

assigned values of 600 m each, and a value of 

0.003 m/d was used for KGHB (Montgomery, 1988). A 

value of 9.14 m was used for bGHB, the average thickness of the upper Hawthorn confining unit reported by 

Knapp and others (1986). The bGHB could not be spatially variable because no published maps of the thickness of this unit could be found. 

The hGHB values were set using contour maps of 

the potentiometric surface of the sandstone aquifer. 

Sufficient water-level measurements were obtained for 

the March 1996 steady stress period, but not for the 

September 1996 period. To approximately define the 

potentiometric surface of the sandstone aquifer, water 

levels at 15 monitoring wells for March 1996 were 

averaged, plotted, contoured, and interpolated to the 

hGHB. For the September 1996 period, average water 

levels at the same 15 monitoring wells were computed, 

plotted, and contoured to define the average potentiometric surface of the sandstone aquifer for the entire 

1996 calendar year. This average surface then was 

interpolated to the hGHB for the September 1996 steadystate stress period.

Recharge

The Recharge and Evapotranspiration packages 

(McDonald and Harbaugh, 1988) were used to represent net recharge, or the water applied to land surface 

through precipitation that exceeds evapotranspiration 

and runoff and infiltrates through the unsaturated zone 

to the water table. An areal flux [L/T] of rainfall minus 

runoff was applied to the highest active cell in a vertical column of layers using the Recharge package 

(McDonald and Harbaugh, 1988) for both steady-state 

stress periods. Average monthly rainfall rates were 

computed from daily rainfall totals collected in 1996 at 

a rainfall monitoring station near Bonita Springs. The 

average rainfall rate in March 1996 was about 0.3 

cm/d, and the average rainfall rate in September 1996 

was about 0.4 cm/d. Runoff was estimated using landuse coefficients developed during previous investigations of the study area (Bennett, 1992). These land-use 

coefficients suggest 10 to 30 percent of rainfall is converted to runoff during a storm event. The average of 

land-use coefficients (about 0.127) reported by Bennett (1992) was used to estimate the amount of rainfall 

lost to runoff during March and September 1996. A 

weighted average based on the individual area of each 

land-use type within the total area of the active model 

domain could not be computed because runoff coefficients for recent land-use maps were not available. As 

previously mentioned, runoff was subtracted from 

rainfall, and the resulting areal flux [L/T] was input 

for both steady-state stress periods in the Recharge 

package.

Evapotranspiration was simulated using the 

Evapotranspiration package (McDonald and Harbaugh, 1988), with the evapotranspiration surface set 

to land surface and an extinction depth set to 5 m at all 

active cells in the model domain. As previously 

defined, setting extinction depths to 5 m is a reasonable compromise given the current understanding of 

the evapotranspiration process in Florida. Maximum 

evapotranspiration rates were uniformly set across the 

model grid using free surface pan evaporation data 

(fig. 25) and a pan multiplier of 0.7 (Swancar and 

QGHB CONDGHB hGHB hijk , , = ( ) –

CONDGHB

LwKGHB

bGHB

= ---------------------
42 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

others, 2000, p. 42). This means maximum groundwater evapotranspiration rates in the Evapotranspiration package were 70 percent of the average March 

1996 pan evaporation rate for the first steady-state 

stress period and 70 percent of the average September 

1996 pan evaporation rate for the second steady-state 

stress period.

Wells

The Well package (McDonald and Harbaugh, 

1988) was used to represent the withdrawal of ground 

water from the water-table and lower Tamiami aquifers. Well boundaries may be used for injection or 

extraction wells. The layer, row, column, and injection 

or extraction volumetric rate are required input for the 

Well package. The location and volumetric rate of 

monitored ground-water pumpage were obtained from 

the SFWMD and translated to the appropriate layer, 

row, column, and stress period of the model simulation. Well construction information, such as depth to 

the top of casing and depth to the bottom of the openhole interval, was used to determine the appropriate 

model layer in which to place the pumping well. Areas 

near Bonita Springs that are most likely to withdraw 

large quantities of unmonitored ground water from the 

lower Tamiami aquifer also were delineated (fig. 26) 

with the help of SFWMD personnel. Because this 

pumpage was unmonitored, volumetric rates and openhole intervals were not known. The missing pumpage 

rates were set approximately equal to the monitored 

ground-water pumpage in March and September 1996. 

Although this may seem arbitrary, these missing 

pumpage rates were better estimated with UCODE 

during model calibration. The open-hole intervals of 

unmonitored pumping wells were assumed to be 

present at the center of the lower Tamiami aquifer 

(pl. 2).

Parameter Estimation

The model was calibrated with the UCODE 

parameter estimation routine (Poeter and Hill, 1998) to 

41 average monthly head and 2 average monthly flow 

observations computed from USGS monitoring wells 

and gaging stations, respectively, in the study area 

(table 4). Each head observation is the average of all 

measurements made at the well during March or September 1996. Flow observations are the sum of the 

average of all flow measurements made at the northern 

and southern branches of the Estero River, Spring 

Creek, and Imperial River during March and September 1996.

PAN EVAPORATION,

IN CENTIMETERS

1996

10

12

14

16

18

20

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

0

2

4

6

8

Figure 25. Total monthly pan evaporation from several monitoring stations near Bonita Springs.
Simulation of Saltwater Intrusion near Bonita Springs 43

With UCODE, head and flow observations were 

weighted and summed together with their simulated 

equivalents as the objective function, S(b),

, (15)

where b is a vector containing values of each of the 

parameters being estimated, nh is the number of head 

observations, wi

 is the weight for the ith observation, hi

is the ith observed water level, hi

’(b) is the simulated 

equivalent of the ith observed water level (a function 

of b), nq is the number of flow observations, qi is the 

ith observed flow, and qi

’(b) is the simulated equivalent of the ith observed flow (a function of b). UCODE 

solved for parameter values, b, that minimize this 

objective function, S(b), resulting in simulated heads 

and flows that most closely resemble their observed 

equivalents.

ZONE

1 ZONE

2

ZONE

3

ZONE

4

ZONE

5

846

41

41

75

GULF OF MEXICO

Bonita

Springs

LEE COUNTY

COLLIER COUNTY

Cocohatchee River

Spring Creek

Imperial River

81°52 ́30 ́ ́ 81°45 ́00 ́ ́

26°22 ́30 ́ ́

26°15 ́00 ́ ́

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

0

0

6 MILES

6 KILOMETERS

2 4

2 4

AREA MOST LIKELY TO

WITHDRAW LARGE

QUANTITIES OF

UNMONITORED

GROUND WATER

ZONE

1

EXPLANATION

Figure 26. Areas with potentially large quantities of unmonitored ground-water pumpage.

S b( ) wi hi hi [ ] – ′ ( ) b 2

i = 1

nh

∑

wi

i = 1

nq

∑ qi qi [ ] – ′ ( ) b 2

= +
44 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

Table 4. Observations used during model calibration to March and September 1996

conditions 

[Aquifer: LT, lower Tamiami aquifer; WT, water-table aquifer. USGS, U.S. Geological Survey; N/A, not applicable; 

QWDATA,Water Quality Database; ADAPS, Automated Data Processing System]

Site

identification

Universal transverse 

mercator coordinates

Aquifer Date

Average 

head 

(meters) or 

flow1

USGS

data

x source

(meters)

y

(meters)

C-304 439677 2906341 LT Mar. 96 1.6 QWDATA

C-391 420917 2897007 LT Mar. 96 .8 ADAPS

C-391 420917 2897007 LT Sept. 96 1.4 ADAPS

C-392 420945 2897038 WT Mar. 96 1.9 ADAPS

C-392 420945 2897038 WT Sept. 96 2.1 ADAPS

C-430 423141 2897516 LT Mar. 96 1.2 QWDATA

C-460 421614 2902048 LT Mar. 96 1.5 ADAPS

C-460 421614 2902048 LT Sept. 96 2 ADAPS

C-489 420885 2900914 LT Mar. 96 .5 QWDATA

C-492 439638 2917334 WT Mar. 96 5 ADAPS

C-492 439638 2917334 WT Sept. 96 5.3 ADAPS

C-516 419370 2896761 LT Mar. 96 1.3 QWDATA

C-528 419240 2898054 LT Mar. 96 .6 QWDATA

C-951 441394 2901359 LT Mar. 96 1.1 ADAPS

C-951 441394 2901359 LT Sept. 96 1.9 ADAPS

C-953 441394 2901359 WT Mar. 96 1.9 ADAPS

C-953 441394 2901359 WT Sept. 96 2.4 ADAPS

C-977 435395 2892948 LT Mar. 96 1.7 QWDATA

C-978 440163 2915292 WT Mar. 96 4.8 QWDATA

C-979 440163 2915292 LT Mar. 96 4 QWDATA

C-997 431283 2904547 WT Mar. 96 2.6 ADAPS

C-1004 422249 2906167 LT Sept. 96 1.1 ADAPS

C-1059 419870 2905711 WT Mar. 96 1.9 QWDATA

C-1061 420058 2900325 WT Mar. 96 3.9 QWDATA

C-1083 421307 2910911 LT Mar. 96 .5 ADAPS

C-1083 421307 2910911 LT Sept. 96 1 ADAPS

FLOW1 N/A2 N/A2 N/A Mar. 96 40,394 ADAPS

FLOW2 N/A2 N/A 2 N/A Sept. 96 271,820 ADAPS

L-738 422405 2913580 LT Mar. 96 0 ADAPS

L-738 422405 2913580 LT Sept. 96 .9 ADAPS

L-1964 439799 2938122 WT Mar. 96 7.2 QWDATA

L-1999 427787 2932554 WT Mar. 96 6.3 QWDATA

L-2195 427917 2912841 WT Mar. 96 2.8 ADAPS

L-2195 427917 2912841 WT Sept. 96 3.6 ADAPS

L-2308 418735 2923716 WT Mar. 96 3.4 QWDATA

L-2550 430872 2926115 WT Mar. 96 3.3 ADAPS

L-2550 430872 2926115 WT Sept. 96 3.7 ADAPS

L-5722 422255 2914834 WT Mar. 96 1.9 QWDATA

L-5723 422255 2914834 LT Mar. 96 -1.3 QWDATA

L-5724 418350 2912528 WT Mar. 96 2.6 QWDATA

L-5725 418332 2912489 LT Mar. 96 0 QWDATA

L-5730 418739 2919993 WT Mar. 96 3.4 QWDATA

L-5747 421417 2918346 LT Mar. 96 -1.3 QWDATA 1

Flow shown in meters per day.

2

Flow observations are the sum of estimated flows at four different locations. These locations (northern and 

southern branches of the Estero River, Spring Creek, and Imperial River) are discussed in the text.
Simulation of Saltwater Intrusion near Bonita Springs 45

With UCODE, observations are weighted to: 

(1) maintain similar dimensions in the objective function so that head and flow residuals can be added 

together, and (2) emphasize observations that are more 

or less important to “best fit” with regression. For 

example, weights can be used to make an accurately 

measured observation more important to “best fit” and 

an inaccurately measured observation less important to 

“best fit.” For this study, observations were weighted 

to: (1) account for the potential measurement error, 

such as the top-of-casing elevation, down-to-water distance, or instrumentation drift; and (2) account for the 

potential deviance from the true monthly average.

The variance statistic was used to compute 

observation weights. A variance was assigned to each 

observation well to account for the potential measurement errors described above. For convenience, the 

variance statistic was also used to account for the 

potential deviance from the true monthly average. This 

was accomplished by summing the squared differences 

of individual water-level measurements from the 

monthly average and dividing by one less than the total 

number of measurements. At monitoring wells where 

only one or two depth-to-water measurements were 

taken during March or September 1996, the average 

variance of water-level measurements at observation 

wells with continuous monitoring equipment was 

used. Using the variance statistic was convenient 

because variances are additive, and the final variance 

statistic used to calculate observation weights was the 

sum of the variance that accounts for measurement 

error and the variance that accounts for the potential 

deviance from the true monthly average.

A primary objective of this project was to accurately simulate saltwater movement in the lower Tamiami aquifer beneath Bonita Springs. To accomplish 

this objective, accurately simulated water levels in this 

area of the aquifer were necessary. Unmonitored 

ground-water pumpage created large cones of depression in the lower Tamiami aquifer beneath Bonita 

Springs. Large discrepancies between observed and 

simulated water levels initially were present in this 

area of the model because simulated water levels cannot be drawn down where pumpage data do not exist. 

For this reason, zones of unmonitored ground-water 

pumpage with “sufficient” sensitivity were estimated 

with UCODE, resulting in simulated water levels that 

closely match observed water levels in important areas 

near the coast. 

Zones of unmonitored ground-water pumpage 

were determined to have “sufficient” sensitivity 

by examining composite-scaled sensitivities. 

Composite-scaled sensitivities (CSS) computed by 

UCODE are measures of the amount of information 

that all the observations provide for estimating a 

parameter (Poeter and Hill, 1998). If a parameter has 

large CSS, then the observations hold much information for estimating that parameter value. If a parameter 

has small CSS, then the observations hold little information for estimating that parameter value. Given this 

relation, the regression performed by UCODE can 

have difficulty converging on estimates of parameters 

with small CSS, because very little information is provided by the observations for estimating these parameter values. Thus, only parameters with large or 

“sufficient” CSS that are not correlated are successfully estimated with UCODE.

Parameter correlation is measured using correlation coefficients calculated as the covariance between 

two parameters divided by the product of their standard deviations (Hill, 1998). Correlation coefficients 

range from 1.0 to -1.0, with values close to 1.0 or -1.0 

indicating parameters that are correlated or inversely 

correlated, but cannot be estimated uniquely with the 

observations used to calibrate a flow model. It is 

already well established that hydraulic conductivity 

parameters (K) and flow parameters (Q) representing, 

for example, recharge or ground-water pumpage are 

often highly correlated when calibrating a flow model 

using only hydraulic-head observations (Hill, 1998). 

Adding flow observations, such as streamflow gains 

and losses, can greatly reduce correlation of K and Q

parameters and result in unique parameter estimates.

Unmonitored ground-water pumpage was correlated with the most sensitive model parameters (evapotranspiration extinction depth, canal conductance, and 

vertical hydraulic conductivities of the Tamiami confining beds and upper Hawthorn confining unit) as 

indicated by CSS (fig. 27) and correlation coefficients. 

This meant that while trying to estimate unmonitored 

ground-water pumpage with more sensitive model 

parameters, unique estimates could not be found. This 

is a disadvantage because: (1) a better regional model 

fit probably could be attained if the more sensitive 

model parameters could be estimated; and (2) some of 

the more sensitive model parameters, such as hydraulic conductivities and canal conductances, probably 

are more uncertain than the unmonitored ground-water 

pumpage. However, unique UCODE estimates of 
46 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

unmonitored ground-water pumpage zones with sufficient sensitivity and pan evaporation multipliers were 

found (table 5). The uniqueness is indicated by 

UCODE correlation coefficients (table 6) and the fact 

that final UCODE estimates of these parameters were 

not sensitive to the parameter starting values.

In the stress period representing September 

1996, UCODE estimated a pan evaporation multiplier 

of 1.13, which means the maximum evapotranspiration rate is about 13 percent larger than the pan 

evaporation rate. This parameter estimate probably is 

unreasonable because maximum evapotranspiration 

rates likely are less than pan evaporation rates in most 

circumstances. Unreasonable parameter estimates are 

valuable because they suggest that either the model 

may not accurately represent some aspect of the physical system (most likely), or there may be an error in 

the observations used to calibrate the model (Poeter 

and Hill, 1997).

0 2 4 6 8 10 12 14

UNMONITORED PUMPAGE ZONE 2, MARCH 1996

UNMONITORED PUMPAGE M ZONE 4, ARCH 1996

UNMONITORED PUMPAGE M ZONE 5, ARCH 1996

UNMONITORED PUMPAGE ZONE 4, SEPTEMBER 1996

UNMONITORED PUMPAGE S ZONE 5, EPTEMBER 1996

PAN EVAPORATION MULTIPLIER, MARCH 1996

PAN EVAPORATION MULTIPLIER, SEPTEMBER 1996

HORIZONTAL HYDRAULIC CONDUCTIVITY WATER-TABLE AQUIFER

VERTICAL HYDRAULIC CONDUCTIVITY WATER-TABLE AQUIFER

HORIZONTAL HYDRAULIC CONDUCTIVITY TAMIAMI CONFINING BEDS

VERTICAL HYDRAULIC CONDUCTIVITY TAMIAMI CONFINING BEDS

HORIZONTAL HYDRAULIC CONDUCTIVITY LOWER TAMIAMI AQUIFER

HORIZONTAL HYDRAULIC CONDUCTIVITY LOWER TAMIAMI AQUIFER, CORAL ROCK FACIES

VERTICAL HYDRAULIC CONDUCTIVITY LOWER TAMIAMI AQUIFER

VERTICAL HYDRAULIC CONDUCTIVITY , CORAL ROCK FACIES LOWER TAMIAMI AQUIFER

HORIZONTAL HYDRAULIC CONDUCTIVITY UPPER HAWTHORN CONFINING UNIT

VERTICAL HYDRAULIC CONDUCTIVITY UPPER HAWTHORN CONFINING UNIT

HORIZONTAL HYDRAULIC CONDUCTIVITY SANDSTONE AQUIFER

VERTICAL HYDRAULIC CONDUCTIVITY SANDSTONE AQUIFER

RUNOFF, MARCH 1996

RUNOFF, SEPTEMBER 1996

ESTERO RIVER BED CONDUCTANCE

SPRING CREEK BED CONDUCTANCE

IMPERIAL RIVER BED CONDUCTANCE

KEHL CANAL BED CONDUCTANCE

COCOHATCHEE RIVER BED CONDUCTANCE

CANAL SYSTEM CONDUCTANCE (EAST OF NAPLES)

GENERAL-HEAD BOUNDARY CONDUCTANCE

EVAPOTRANSPIRATION DEPTH

COMPOSITE-SCALED SENSITIVITY, DIMENSIONLESS

ESTIMATED WITH UCODE

NOT ESTIMATED

EXPLANATION

Figure 27. Composite-scaled sensitivities computed while calibrating the model to March and September 1996 

conditions. Parameters with no visible bar have very small or zero sensitivity.
Simulation of Saltwater Intrusion near Bonita Springs 47

In this model, rainfall may be overestimated, 

runoff may be underestimated, hydraulic conductivity 

may be underestimated, or a combination of these and 

other errors may exist. It is also possible that a process 

such as interception that prevents rainfall from reaching the water table is not represented in the model. In 

any of these circumstances, UCODE would estimate a 

larger maximum evapotranspiration to compensate for 

the model error, or the missing process that affects “net 

recharge.” Because maps of “net recharge” looked reasonable and were consistent with field observations by 

Krulikas and Giese (1995), the model error was not 

resolved. Although undesirable, the unresolved error 

was justified because compensating errors in a model 

representing the various processes that affect “net 

recharge” can produce “net recharge” rates that are 

accurate.

When estimating zones of unmonitored groundwater pumpage with sufficient sensitivity and pan 

evaporation multipliers, the nonlinear least-squares 

regression performed by UCODE converged by satisfying the sum of the squares residual criteria, which 

was set to zero difference between parameter estimation iterations. This means the difference in the 

weighted objective function changed by this amount 

over three parameter estimation or regression iterations (Poeter and Hill, 1998). A traditional statistic for 

reporting calibration results is the mean absolute error 

(MAE), which is calculated by taking the average of 

the absolute values of the differences between 

observed and simulated heads. The MAE of the calibration was 0.73 m for the 41 average monthly waterlevel observations in the water-table and lower Tamiami aquifers. Graphical representations of the model 

Table 5. Estimates of unmonitored ground-water pumpage and evapotranspiration 

multipliers with confidence intervals computed by UCODE

[Parameters are in cubic meters per day, except for evaporation multipliers, which are dimensionless]

Parameter Parameter

 estimate

Linear 95-percent 

confidence interval

Unmonitored pumpage zone 1, March 1996 -1,080 275; -2,440

Unmonitored pumpage zone 4, March 1996 -1,180 1,650; -4,010

Unmonitored pumpage zone 5, March 1996 -6.1 4,450; -4,750

Unmonitored pumpage zone 4, September 1996 -352 4,610; -5,310

Unmonitored pumpage zone 5, September 1996 11 8.26; -30

Pan evaporation multiplier, March 1996 .64 0.78; 0.53

Pan evaporation multiplier, September 1996 1.13 1.46; 0.87

Table 6. UCODE estimated parameter correlation coefficients

Parameter

Unmonitored ground-water pumpage Pan evaporation

multiplier

Zone 1

(March 

1996)

Zone 4,

(March

1996)

Zone 5

(March

1996)

Zone 4, 

(September

1996)

Zone 5, 

(September

1996)

March

1996 

September

1996

Unmonitored pumpage zone 1, 

March 1996 

 1.00

Unmonitored pumpage zone 4, 

March 1996 

 -.15 1.00

Unmonitored pumpage zone 5, 

March 1996 

 .03 -.38 1.00

Unmonitored pumpage zone 4, 

September 1996 

 .00 .00 .00 1.00

Unmonitored pumpage zone 5, 

September 1996 

 .00 .00 .00 .00 1.00

Pan evaporation multiplier, 

March 1996 

 .06 .09 .26 .00 .00 1.00

Pan evaporation multiplier

 September 1996 

 .00 .00 .01 .20 .00 .01 1.00
48 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

calibration are made by comparing observed and simulated water levels (fig. 28) and comparing weighted 

residuals and weighted simulated values (fig. 29). The 

comparison of weighted residuals and weighted simulated values is presented because this plot may identify 

model bias or trends that are obscured by the traditional comparison of observed and simulated water 

levels (Mary Hill, U.S. Geological Survey, written 

commun., 1999). No bias or trends are obvious on the 

plot in figure 29. The calibration plots and MAE statistic suggest that the model adequately represents the 

ground-water flow system in the water-table and lower 

Tamiami aquifers near Bonita Springs.

A component of the water budget important to 

water managers is unmonitored ground-water pumpage. Estimates of unmonitored ground-water pumpage 

zone rates per cell with confidence intervals were 

computed by UCODE (table 5). By calibrating to 

March and September 1996 water-level and flow 

observations, the average 1996 rate of unmonitored 

ground-water pumpage is bracketed because the rate 

of unmonitored ground-water pumpage probably is 

greatest during March 1995 and lowest during September 1995, as evidenced by historical ground-water 

withdrawals. The average 1996 rate of unmonitored 

ground-water pumpage can be estimated by: (1) multiplying the UCODE-estimated unmonitored groundwater pumpage rate for each zone by the number of 

model cells in that zone, (2) summing the total rate of 

unmonitored ground-water pumpage in March 1996 

and September 1996, and (3) computing a weighted 

average. The weight was based on a 7 to 5 ratio of dryto wet-season months, a typical southwestern Florida 

water year (Virogroup, Inc., 1993). Using this method, 

the average estimated rate of unmonitored groundwater pumpage from the lower Tamiami aquifer was 

-2

-1

0

1

2

3

4

5

6

7

8

-2 -1 0 1 2 3 4 5 6 7 8

AVERAGE MONTHLY SIMULATED WATER-LEVELS IN MARCH

OR SEPTEMBER 1996, IN METERS RELATIVE TO NGVD 29

AVERAGE MONTHLY OBSERVED WATER LEVELS IN MARCH

OR SEPTEMBER 1996, IN METERS RELATIVE TO NGVD 29

LOWER TAMIAMI AQUIFER

WATER-TABLE AQUIFER

EXPLANATION

Figure 28. Comparison of observed and simulated water levels. NGVD 29 is National Geodetic 

Vertical Datum of 1929.
Simulation of Saltwater Intrusion near Bonita Springs 49

about 52,000 m3/d in 1996. The average estimated rate 

of monitored ground-water pumpage from the watertable and lower Tamiami aquifers in the active model 

domain (pl. 2) was about 59,000 m3

/d in 1996.

A good exercise to test a simulated water budget 

is to compare simulated net recharge with estimates of 

net recharge as computed by other methods. Krulikas 

and Giese (1995) used the chloride concentration ratio 

and flow-tube analysis methods to estimate net 

recharge, which ranged from 0 to 25 cm/yr in the study 

area in 1989. Net recharge maps were prepared for 

March and September 1996 using the cell-by-cell flow 

output from the MODFLOW simulations (figs. 30 and 

31, respectively). Simulated values of net recharge 

generally ranged from 0 to 38 cm/yr. Net discharge 

occurred in small areas as indicated by the negative 

values shown in figures 30 and 31. One possible explanation for the difference in maximum net recharge 

rates (field-data computed compared to simulated) is 

that lowering the water table through pumping 

between 1989 and 1996 could have induced recharge 

rates in excess of 25 cm/yr (Krulikas and Giese, 1995). 

Topography appears to influence the distribution 

of net recharge. In fact, simulation results suggest that 

there are localized areas in the water-table aquifer 

where net discharge occurs because of the close proximity of the water table to land surface and the effects 

of evapotranspiration (figs. 30 and 31). These net discharge areas constitute about 5 percent of the active 

model domain. Net recharge is greater in areas where 

land surface is relatively higher, such as Bonita 

Springs and areas south of the Caloosahatchee River 

along the Immokalee Rise (figs. 1 and 2), because the 

water table is deeper below land surface and the effects 

of evapotranspiration are less. The resulting net 

recharge maps (figs. 30 and 31) reflect the local topography. 

Simulated water levels, streamflows, and net 

recharge rates adequately represent the empirical data 

describing these hydrologic processes in the study 

area. However, adequate representation of selected 

parameters does not ensure the accuracy of predictive 

simulations (Konikow and Bredehoft, 1992). Nevertheless, calibration or history matching is necessary to 

reduce errors in the conceptual model and to obtain 

more representative parameter values, given the scale 

of simulation.

Confidence Intervals

Linear 95-percent confidence intervals are one 

of the many useful statistics computed by UCODE 

during model calibration. Linear 95-percent confidence intervals were calculated for each UCODE estimated parameter, and represent a range that has a 

95-percent probability of containing the true value if 

the model correctly represents the true ground-water 

flow system (Hill, 1998). The width of a confidence 

interval can be thought of as a measure of the likely 

precision of the parameter estimate. Wide intervals 

indicate less precision or more uncertainty, and narrow 

intervals indicate more precision or less uncertainty.

The linear 95-percent confidence intervals are 

wide for estimates of unmonitored ground-water 

pumpage and narrow for estimates of pan evapotranspiration multipliers (table 5). This suggests unmonitored ground-water pumpage rates are estimated with 

-20

-15

-10

-5

0

5

10

15

20

-20 0 20 40 60 80 100 120

WEIGHTED SIMULATED VALUE

WEIGHTED RESIDUAL

WATER-TABLE AQUIFER

FLOW OBSERVATION

LOWER TAMIAMI AQUIFER

EXPLANATION

Figure 29. Comparison of weighted residuals and weighted simulated values.
50 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

INACTIVE CELLS

NO FLOW BOUNDARY

CONDITION REPRESENTING

GROUND-WATER DIVIDES

846

41

41

82

884

75

29

GULF OF MEXICO

Fort

Myers

Lehigh

Acres

Naples

Bonita

Springs

Immokalee

Cape

Coral

Sanibel Island

LEE CO

HENDRY COUNTY

COLLIER COUNTY

COLLIER CO

Trail

Tamiami

Alligator Alley

Tamiami Canal

Estero Bay

Ca a loos hatchee River

82°00 ́ 81°45 ́ 81°30 ́

26°30 ́

26°15 ́

26°00 ́

EXPLANATION

NET RECHARGE, IN

CENTIMETERS

PER YEAR

0

0

7.5 MILES

7.5 KILOMETERS

2.5 5

Base from U.S. Geological Survey digital data, 1:100,000 2.5 5

Universal Transverse Mercator projection, zone 17

-13 to <0

0 to <13

13 to <25

25 to 38

Figure 30. Simulated net recharge to the water-table aquifer during March 1996.
Simulation of Saltwater Intrusion near Bonita Springs 51

846

41

41

82

884

75

29

GULF OF MEXICO

Fort

Myers

Lehigh

Acres

Naples

Bonita

Springs

Immokalee

Cape

Coral

Sanibel Island

LEE CO

HENDRY COUNTY

COLLIER COUNTY

COLLIER CO

Trail

Tamiami

Alligator Alley

Tamiami Canal

Estero Bay

Ca a loos hatchee River

82°00 ́ 81°45 ́ 81°30 ́

26°30 ́

26°15 ́

26°00 ́

NET RECHARGE, IN

CENTIMETERS

PER YEAR

INACTIVE CELLS

EXPLANATION

NO FLOW BOUNDARY

CONDITION REPRESENTING

GROUND-WATER DIVIDES

0

0

7.5 MILES

7.5 KILOMETERS

2.5 5

Base from U.S. Geological Survey digital data, 1:100,000 2.5 5

Universal Transverse Mercator projection, zone 17

-13 to <0

0 to <13

13 to <25

25 to <38

Figure 31. Simulated net recharge to the water-table aquifer during September 1996.
52 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

less precision or more uncertainty than pan evaporation 

multipliers. The design of the USGS network of monitoring wells and streamgaging stations in the study area 

apparently provides more information on estimating 

pan evaporation multipliers than estimating unmonitored ground-water pumpage rates. Conversely, one 

could also say that the pan evaporation parameters 

explain more of the water-level and discharge data 

monitored by the network than does the unmonitored 

ground-water pumpage.

Potential Movement of Saltwater to 

Equilibrium with Typical, Modern, and 

Seasonal Stresses

The predevelopment distribution of saltwater 

was moved to dynamic equilibrium with calibrated 

March and September 1996 steady-state stress periods 

(table 2). The same spatial discretization and aquifer 

properties previously described were used. Calibrated 

March and September 1996 steady-state stress periods 

were run successively until saltwater entering and leaving the simulation was about equal, which took about 

600 years, or 1,200 steady-state stress periods. The 

SEAWAT program (Guo and Langevin, 2002) was 

used with initial conditions that consisted of the final 

distribution of freshwater equivalent head and salinity 

in the predevelopment simulation. The primary use of 

this simulation was to provide a baseline 

scenario to help identify potential mechanisms of saltwater intrusion and to estimate the potential extent of saltwater 

intrusion at dynamic equilibrium with 

modern stresses. Sensitivity analyses 

were performed for each of these primary 

objectives using the baseline scenario 

results.

In general, boundary conditions for 

this simulation are the same as boundary 

conditions used while calibrating the 

model to March and September 1996 

conditions (pl. 2). The UCODE estimates 

of unmonitored ground-water pumpage 

and pan evaporation multipliers (table 5) 

were used. Some minor modifications to 

boundary conditions were necessary.

• Conversion of constant-head 

boundary conditions (pl. 2) to 

freshwater equivalent constanthead boundary conditions in the 

Gulf of Mexico, and conversion of the external 

heads of general-head boundary conditions 

(pl. 2) to freshwater equivalent external heads 

at the base of the lower Tamiami aquifer. These 

conversions were accomplished using chloride 

concentrations measured in the field, where 

data were available. 

• Assignment of a constant concentration boundary equal to 35 kg/m3

 (salinity of seawater) in 

layer 1 at the Gulf of Mexico (pl. 2).

• Assignment of a constant-concentration boundary equal to 35 kg/m3

 (salinity of seawater) in 

layers 2 to 15 at the westernmost column (column 1) in and beneath the Gulf of Mexico 

(pl. 1). 

• Removal of the no-flow boundary condition 

used in layers 2 to 15 to represent the saltwater 

interface (pl. 2). 

• Assignment of a salinity value of 0.3 kg/m3 to 

water entering the model from the general-head 

boundary condition at the base of the lower 

Tamiami aquifer. This was based on limited 

water-quality data from the sandstone aquifer 

(pl. 2).

Baseline Scenario

The final distribution of saltwater in the baseline 

scenario (fig. 32) supports two mechanisms of 

2.70x1011

2.75x1011

2.80x1011

2.85x1011

2.90x1011

2.95x1011

3.00x1011

3.05x1011

0 200 400 600

YEARS

TOTAL SALT MASS IN MODEL

DOMAIN, IN KILOGRAMS

Figure 32. Total salt mass in the model while moving the simulated 

predevelopment distribution of saltwater to equilibrium with calibrated 

March and September 1996 conditions.
Simulation of Saltwater Intrusion near Bonita Springs 53

saltwater intrusion in the lower Tamiami aquifer near 

Bonita Springs—upconing and lateral saltwater intrusion. Results from the baseline scenario were used to 

estimate the potential extent of saltwater intrusion in 

the lower Tamiami aquifer beneath Bonita Springs. 

Mechanisms of Saltwater Intrusion

The baseline scenario suggests that upconing of 

saltwater is the most predominant saltwater intrusion 

mechanism in the lower Tamiami aquifer beneath 

Bonita Springs (figs. 33 and 34). Predominance was 

AREA AFFECTED BY LATERAL SALTWATER

INTRUSION IN MODEL SIMULATION

AREA AFFECTED BY LATERAL SALTWATER

INTRUSION AND UPCONING IN MODEL SIMULATION

AREA AFFECTED BY UPCONING IN MODEL SIMULATION

AREA ENCOMPASSING BONITA SPRINGS. THE

EXTENT AND RATE OF SALTWATER INTRUSION

WERE CALCULATED WITHIN THIS AREA

0

0

6 MILES

6 KILOMETERS

2 4

2 4

81°45 ́

26°30 ́

26°15 ́

GULF OF MEXICO

LEE

COUNTY

COLLIER

COUNTY

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17 EXPLANATION

Bonita

Springs

75

41

Cocohatchee

River

Imperial

River

Estero

River

Spring

Creek

Figure 33. Simulated potential extent of saltwater intrusion in the lower Tamiami aquifer from the baseline scenario 

near Bonita Springs.
54 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

established by identifying the saltwater intrusion 

mechanism that affected the largest inland area. A 

plume of higher salinity ground water (encompassing 

about 100 km2

 in area) developed in the lower Tamiami aquifer near Bonita Springs after 600 years of 

transport with calibrated March and September 1996 

conditions. By animating model results, it was determined that this plume of higher salinity ground water 

developed from upconing of ground water in the 

underlying sandstone aquifer. 

The baseline scenario suggests that lateral 

encroachment is the second most predominant mechanism of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs (figs. 33 and 34). The saltwater 

interface in the lower Tamiami aquifer near Bonita 

Springs moved about 1.5 km inland from its predevelopment position after 600 years of transport with calibrated March and September 1996 conditions. As a 

result, the rate of lateral encroachment averaged about 

2.5 m/yr, which is the total distance that the saltwater 

interface moved (1.5 km) divided by the time length of 

the baseline scenario (600 years). Additionally, the 

baseline scenario suggests that lateral encroachment 

into the potentiometric depression (fig. 22) of the 

lower Tamiami aquifer was hindered by the presence 

of a ground-water mound located between the saltwater interface and the potentiometric depression 

(fig. 35). This simulated ground-water mound seems 

to force freshwater toward the coast and prevents further lateral encroachment. No field data are available, 

however, to verify the presence of the simulated 

mound. If it were shown through the collection of 

additional field data that the mound does not exist, 

then the effects of lateral encroachment may be much 

more severe.

Extent of Saltwater Intrusion

The extent of saltwater intrusion is defined 

herein as the areal or volumetric movement of saltwater from the predevelopment distribution to equilibrium with calibrated March and September 1996 

conditions. Because Bonita Springs was the main area 

of concern, the extent of saltwater intrusion was computed for a subarea centered on Bonita Springs 

(fig. 33). The baseline scenario suggests that the areal 

extent of saltwater intrusion at the base of the lower 

Tamiami aquifer is about 100 km2

, and the volumetric 

extent of saltwater intrusion is about 70,000 hectaremeters (fig. 34). The volumetric extent of saltwater 

intrusion was computed using the same subarea, vertically extended only through the active cells representing the lower Tamiami aquifer. Within this volume, the 

extent of saltwater intrusion was computed by: 

(1) summing the volume of cells in the lower Tamiami 

EXPLANATION

0 17.5 35

SALINITY OF GROUND WATER,

IN KILOGRAMS PER CUBIC METER

SALTWATER INTERFACE

UPCONING PLUMES GULF OF MEXICO

N

Figure 34. The saltwater interface and upconing plumes near equilibrium with calibrated March and 

September 1996 conditions in the water-table and lower Tamiami aquifers near Bonita Springs. The 

upconing plumes are directly beneath Bonita Springs. All shaded areas represent active model cells. 

Blue and tan cells are equivalent in salinity; the tan shading is used to highlight the topography of the 

base of the lower Tamiami aquifer.
Simulation of Saltwater Intrusion near Bonita Springs 55

aquifer with a salinity value less than the potable limit 

(a chloride concentration of about 250 mg/L) in the 

predevelopment distribution of saltwater, (2) summing 

the volume of cells in the lower Tamiami aquifer with 

salinity values less than the potable limit in the baseline scenario, and (3) subtracting the two volumes to 

isolate a volumetric extent of saltwater intrusion.

Areas affected by saltwater intrusion in the baseline scenario (fig. 33) coincide with areas that seem to 

be affected by saltwater intrusion as indicated in isochlor maps created by Schmerge (2001) and Knapp and 

others (1986) (fig. 16). There are discrepancies, however, between simulated salinities and salinities measured at the same location in the field. Possible causes 

of these discrepancies include that: (1) dispersive solute transport was not performed in the baseline scenario, and (2) the model was run for 600 years with 

1996 conditions, so the results cannot be directly compared.

Sensitivity Analyses

The two goals of sensitivity analysis were to: 

(1) determine how uncertainty in the baseline scenario 

parameters and boundary conditions may affect solution results, and (2) determine which parameters and 

boundary conditions in the baseline scenario are most 

important to solution results. The phrase “important to 

solution results” is used rather than “influential on 

solution results” because influence has a specific statistical meaning related more to the occurrence and 

leverage characteristics of the observations used 

-1

0

1

2

3

4

5

0 10 15 25 5 20 30 35 40 45 50 55

MODEL COLUMN NUMBER

EXPLANATION

LAYER 1

LAYER 2

LAYER 3

LAYER 4

LAYER 5

LAYER 6

LAYER 7

LAYER 8 MOUND OF

GROUND WATER

MOUND

FRESHWATER EQUIVALENT

GROUND-WATER HEAD, IN METERS

ABOVE AND BELOW NGVD 29

Bonita

GULF OF MEXICO

Springs

82°00 ́ 81°45 ́ 81°30 ́

26°30 ́

26°15 ́

26°00 ́

0

0

15 MILES

15 KILOMETERS

5 10

5 10

A A ́

Figure 35. Cross section A-A’ along row 40 of the model grid showing a mound of ground water between the 

cones of depression in the lower Tamiami aquifer and the saltwater interface.
56 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

during calibration. Solution results of primary interest 

for this study include mechanisms of saltwater intrusion and the potential extent of saltwater intrusion. 

Thus, sensitivity analyses were performed for both of 

these primary objectives. If the primary objective 

could be easily quantified, the two goals of sensitivity 

analyses (as stated above) were more completely 

accomplished. For example, the extent of saltwater 

intrusion was relatively easy to quantify in three 

dimensions; therefore, parameter values and parameter 

uncertainties most important to this primary objective 

were more easily determined.

Mechanisms of Saltwater Intrusion

The sensitivity analyses, described in this section, were used to help identify mechanisms of saltwater intrusion into the lower Tamiami aquifer near 

Bonita Springs. Identifying predominant mechanisms 

of saltwater intrusion was a difficult objective to quantify; therefore, sensitivity analysis scenarios were 

designed instead of using quantitative predictionscaled sensitivities (PSS) as done by Poeter and Hill 

(1998). Two sensitivity analysis scenarios were 

designed to evaluate potential mechanisms of saltwater 

intrusion without changing simulated heads and flows 

from their calibrated values. It is important to maintain 

simulated heads and flows near observed values during 

sensitivity analyses so the model still resembles the 

system being studied (Poeter, 2001).

Sensitivity scenario 1 was designed to test the 

potential for downward leakage of saltwater from a 

tidal canal or river. This scenario was conducted by 

modifying the river boundary conditions in the baseline scenario. Unlike the baseline scenario, river cells 

(pl. 2) in sensitivity scenario 1 were assigned a salinity 

value of 35 kg/m3

 in an area about 8 to 16 km inland 

from the Gulf of Mexico along Spring Creek and the 

Estero, Imperial, and Cocohatchee Rivers (fig. 1). The 

MAE of water-level observations was 0.73 m using 

this modification to river-cell boundary conditions, 

which is the same as the MAE of water-level observations without this modification to river-cell boundary 

conditions. This suggests the modification did not substantially change heads from their calibrated values. 

Additionally, simulated baseflow was not substantially 

different from its calibrated value.

After 600 years of advective solute transport 

with the river modification noted above, the distribution of saltwater in the lower Tamiami aquifer is identical to the distribution of saltwater in the lower 

Tamiami aquifer in the baseline scenario (fig. 36). 

Ground-water flow vectors in sensitivity scenario 1 

suggest baseflow from the aquifer to the river prevents 

salt in the river cells from leaking downward into the 

water-table aquifer and, subsequently, into the lower 

Tamiami aquifer. Field reconnaissance along the headwaters of the Imperial River during July 2000 confirmed the presence of baseflow from the aquifer to the 

Imperial River. Gaging stations on the northern and 

southern branches of the Estero River, Spring Creek, 

and Imperial River record flow during the dry season, 

suggesting ground water flows from the water-table 

aquifer to these rivers or creeks even in times of 

drought.

Sensitivity scenario 2 was designed to test a 

leaky artesian well as a mechanism of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs. 

This scenario was conducted by modifying a generalhead boundary condition at the base of the lower 

Tamiami aquifer in one model cell (layer 7, row 38, 

column 28). This single general-head boundary cell 

was modified to represent a hypothetical leaky artesian 

ground-water well with connection to the lower 

Hawthorn producing zone (fig. 3). The external head 

for the general-head boundary was set to the average 

of water levels measured in well L-2310 (fig. 36) 

(Prinos and others, 1996). This well has casing that is 

open to the lower Hawthorn producing zone. During 

March and September 1996, average water levels in 

well L-2310 were adjusted for freshwater equivalence 

using a salinity value of 1.3 kg/m3

, which is similar to 

salinity values measured in the lower Hawthorn producing zone by Reese (2000). The conductance term 

for the general-head boundary was computed using the 

area of a 0.1-m diameter well and a hydraulic conductivity of 1 x 106

 m/d. The high value of hydraulic conductivity is considered large enough to transmit water 

(like a leaky well would transmit) from the lower 

Hawthorn producing zone to the base of the lower 

Tamiami aquifer. The salinity of the general-head 

boundary, representing the lower Hawthorn producing 

zone locally, was set to the salinity value used to adjust 

average measured water levels for freshwater equivalence (1.3 kg/m3

). The MAE of water-level observations was 0.71 m using this modification to represent a 

leaky well, which was slightly less than the MAE to 

water-level observations without this modification.

The distribution of saltwater at the base of the 

lower Tamiami aquifer, after 600 years of advective 

solute transport using the general-head boundary mod-
Simulation of Saltwater Intrusion near Bonita Springs 57

ification noted above, is quite different than the distribution of saltwater at the base of the lower Tamiami 

producing zone in the baseline scenario (fig. 36). The 

different distribution of salinity suggests leaky wells 

with connection to the lower Hawthorn producing 

zone could transport large amounts of saltwater into 

areas of the lower Tamiami aquifer previously 

occupied by fresher ground water. These results probably have a physical basis because leaky artesian wells 

have often been identified as sources of saltwater 

intrusion in shallow aquifers in southwestern Florida 

(Burns, 1983; Schmerge, 2001).

Extent of Saltwater Intrusion

The sensitivity analyses, described in this section, were used to help identify the parameters and 

parameter uncertainties most important to estimating 

the extent of saltwater intrusion in the lower Tamiami 

aquifer near Bonita Springs (table 7). Estimating the 

extent of saltwater intrusion was an objective relatively 

easy to quantify; therefore, quantitative PSS (Hill, 

1998) were computed instead of using sensitivity analysis scenarios. PSS allow parameters to be ranked by 

relative importance. Furthermore, the contribution of 

uncertainty in a given input parameter to the overall 

uncertainty in a model increases as sensitivity coefficients increase (Zheng and Bennett, 2002). As discussed, these sensitivity coefficients are analogous to 

the PSS that are computed and described herein. Thus, 

the main contributors of uncertainty to estimating the 

extent of saltwater intrusion in the lower Tamiami 

aquifer may be identified by the PSS presented herein.

846

41

GULF OF MEXICO

Bonita

Springs

LEE COUNTY

COLLIER COUNTY

Cocohatchee River

Spring Creek

Imperial River

81°52 ́30 ́ ́ 81°45 ́00 ́ ́

26°22 ́30 ́ ́

26°15 ́00 ́ ́

0

0

6 MILES

6 KILOMETERS

2 4

2 4

Saltwater

Brackish

water

Freshwater

L-2310

LOCATION OF LEAKY ARTESIAN WELL--In

sensitivity scenario 2

EXPLANATION

POSITION OF THE SALTWATER INTERFACE--

At the base of the lower Tamiami aquifer in the

baseline scenario and sensitivity scenario 1

AREA OF THE AQUIFER AFFECTED BY

SALTWATER INTRUSION--Through a

hypothetical leaky artesian well in sensitivity

scenario 2

Base from U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, zone 17

Figure 36. Distribution of saltwater at the base of the lower Tamiami aquifer at the end of 

the baseline scenario, and two sensitivity analysis scenarios.
58 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

The PSS were calculated using the volumetric 

estimate rather than the areal estimate of the extent of 

saltwater intrusion in the baseline scenario. The volumetric estimate is more representative of the extent of 

saltwater intrusion that would be observed in the real 

ground-water flow system near Bonita Springs (saltwater moves in three dimensions). The PSS were computed as:

PSS = (∆PD/∆B)(B/100)(100/PD), (16)

where ∆PD is the change in the volumetric extent of 

saltwater intrusion from the baseline scenario, ∆B is 

the perturbation of the parameter; B is the parameter 

value in the baseline scenario; and PD is the volumetric extent of saltwater intrusion in the baseline scenario. The resulting statistic is the ratio of the 

fractional change in the volumetric extent of saltwater 

intrusion after 600 years of transport to the fractional 

change in the parameter value.

Preliminary attempts to save time by shortening 

the time length of the baseline scenario (600 years) 

and, thus, the time length of PSS runs, resulted in zero 

sensitivity for many parameters because the change in 

the volumetric extent of saltwater intrusion (∆PD) 

remained the same as the volumetric extent of saltwater intrusion in the baseline scenario (PD). Additionally, zero sensitivity was computed when parameter 

perturbations (∆B) were too small. In theory, PSS 

should approach an exact value as the size of parameter perturbations (∆B) decrease. However, Poeter and 

Hill (1998) explain that small parameter perturbations 

(∆B) could result in zero sensitivity because, for 

instance, the volumetric extent of saltwater intrusion 

under small parameter perturbations (∆B) could 

remain the same as the volumetric extent of saltwater 

Table 7. Changes made to baseline scenario parameters for a sensitivity analysis of the extent of saltwater intrusion in the 

lower Tamiami aquifer near Bonita Springs

Parameter Change from baseline scenario

Sea level Increased by 5 percent

Elevation of general-head boundary Increased by 5 percent

River bottoms Increased by 5 percent

Runoff for March 1996 Increased by 2.54 centimeters

Potentiometric surface of the sandstone aquifer Increased by 0.5 meters

Ground-water pumpage Increased by 10 percent

Horizontal hydraulic conductivity of water-table aquifer Increased by 700 meters per day

Vertical hydraulic conductivity of Tamiami confining beds Increased by 0.005 meter per day

Runoff September 1996 Increased by 2.54 centimeters

Horizontal hydraulic conductivity, zone 1, lower Tamiami aquifer Increased by 130 meters per day

Salinity in sandstone aquifer Increased by 0.3 kilogram per cubic meter

Horizontal hydraulic conductivity, zone 2, lower Tamiami aquifer (coral 

reef facies)

Increased by 170 meters per day

Vertical hydraulic conductivity of upper Hawthorn

confining unit

Increased by 0.006 meter per day

Horizontal hydraulic conductivity of sandstone aquifer Increased by 80 meters per day

River and canal stages Increased both by 0.05 meter per day

River salinity Increased by 0.3 kilogram per cubic meter

Pan evaporation multiplier for March 1996 Increased by 5 percent

Pan evaporation multiplier for September 1996 Increased by 5 percent

General-head boundary conductance Increased by 100 percent

River conductance Increased by 100 percent

Effective porosity Decreased by 20 percent
Simulation of Saltwater Intrusion near Bonita Springs 59

intrusion in the baseline scenario (PD) within the 

accuracy of the model output. Under this circumstance, ∆PD in equation 16 would be equal to zero, 

resulting in zero sensitivity. Conversely, parameter 

perturbations (∆B) that are too large can result in inaccurate PSS because simulated heads and flows may no 

longer be near observed values, resulting in a model 

that no longer resembles the system being studied. 

Parameter perturbations listed in table 7 were determined based on this information.

Because the equations used to compute the 

extent of saltwater intrusion may be nonlinear with 

respect to some parameter values, the PSS can change 

with different perturbations in the parameter value 

(∆B) (table 7). This change is described in more general terms by Zheng and Bennett (2002). Based on this 

description, it is also expected that if the equations 

used to estimate the extent of saltwater intrusion are 

linear with respect to a parameter value (B), then the 

different perturbations in the parameter value (∆B) will 

result in the same PSS.

Apparently, runoff, pan evaporation multipliers, 

ground-water pumpage, sea level and salinity of the 

Gulf of Mexico, and the potentiometric surface of the 

sandstone aquifer (when ranked by PSS) are most 

important for estimating the extent of saltwater intrusion near Bonita Springs (fig. 37). Future efforts at 

estimating the extent of saltwater intrusion in this location would likely benefit from improved representation 

of these stresses and boundary conditions. For simplicity of reporting and communicating results, the sensitivity of evapotranspiration parameters and runoff can 

be grouped into net recharge, although it is notable that 

dry-season sensitivities for these two processes are 

higher than wet-season sensitivities (fig. 37). When 

trying to estimate the extent of saltwater intrusion in 

the lower Tamiami aquifer near Bonita Springs, determination of net recharge during the dry season apparently is more important than during the wet season. 

This may be because the dry season generally comprises more days of a total year than the wet season.

The CSS that were computed during calibration 

(fig. 27) were compared with the PSS that were computed during sensitivity analysis of the extent of saltwater intrusion at equilibrium in the lower Tamiami 

aquifer near Bonita Springs (fig. 37). The CSS of 

evapotranspiration parameters and runoff can be 

grouped together to emphasize that observed 

conditions near Bonita Springs provided the most 

information for estimating net recharge. Additionally, 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

SEA LEVEL

ELEVATION OF GENERAL-HEAD BOUNDARIES

ELEVATION OF RIVER BOTTOMS

RUNOFF IN MARCH 1996

RUNOFF IN SEPTEMBER 1996

POTENTIOMETRIC SURFACE OF THE SANDSTONE AQUIFER

GROUND-WATER PUMPAGE

HORIZONTAL HYDRAULIC CONDUCTIVITY OF THE WATER-TABLE AQUIFER

HORIZONTAL HYDRAULIC CONDUCTIVITY OF THE TAMIAMI CONFINING BEDS

HORIZONTAL HYDRAULIC CONDUCTIVITY OF THE LOWER TAMIAMI AQUIFER

SALINITY OF GROUND WATER IN THE SANDSTONE AQUIFER

HORIZONTAL HYDRAULIC CONDUCTIVITY OF THE LOWER TAMIAMI AQUIFER, CORAL ROCK FACIES

HORIZONTAL HYDRAULIC CONDUCTIVITY OF THE UPPER HAWTHORN CONFINING UNIT

HORIZONTAL HYDRAULIC CONDUCTIVITY OF THE SANDSTONE AQUIFER

VERTICAL HYDRAULIC CONDUCTIVITY OF THE WATER-TABLE AQUIFER

VERTICAL HYDRAULIC CONDUCTIVITY OF THE TAMIAMI CONFINING BEDS

VERTICAL HYDRAULIC CONDUCTIVITY OF THE LOWER TAMIAMI AQUIFER

VERTICAL HYDRAULIC CONDUCTIVITY OF THE LOWER TAMIAMI AQUIFER, CORAL ROCK FACIES

VERTICAL HYDRAULIC CONDUCTIVITY OF THE UPPER HAWTHORN CONFINING UNIT

VERTICAL HYDRAULIC CONDUCTIVITY OF THE SANDSTONE AQUIFER

RIVER AND CANAL STAGES

EFFECTIVE POROSITY

EVAPOTRANSPIRATION DEPTH

PAN EVAPORATION MULTIPLIER IN MARCH 1996

PAN EVAPORATION MULTIPLIER IN SEPTEMBER 1996

GENERAL-HEAD BOUNDARY CONDUCTANCE (BASE OF THE LOWER TAMIAMI AQUIFER)

RIVER AND CANAL BED CONDUCTANCE

SALINITY IN THE GULF OF MEXICO

ABSOLUTE VALUE OF THE PREDICTION-SCALED SENSITIVITY, UNITLESS

FLOW SYSTEM PARAMETER

Figure 37. Sensitivity analysis of the extent of saltwater intrusion. Parameters with no visible bar have very 

small or zero sensitivity.
60 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

the PSS of evapotranspiration parameters and runoff 

(fig. 37) can be grouped together to emphasize net 

recharge as the most important parameter for estimating the extent of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs. The fact that 

observed conditions provided the most information for 

estimating net recharge (the parameter most important 

for estimating the extent of saltwater intrusion in the 

lower Tamiami aquifer near Bonita Springs) is encouraging and supports the idea that calibrated groundwater flow models can adequately simulate saltwater 

movement.

The next most important parameter for estimating the extent of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs is ground-water 

pumpage, as evidenced by the PSS (fig. 37). Water 

level and flow observations used while calibrating the 

model provided relatively little information for estimating unmonitored ground-water pumpage rates as 

evidenced by the CSS (fig. 27), and the width of the 

95-percent confidence intervals on estimates of 

unmonitored ground-water pumpage rates (table 5). 

Future efforts to simulate saltwater movement would 

benefit by improving the representation and estimates 

of ground-water pumpage, because this parameter was 

second in relative importance to estimating the extent 

of saltwater intrusion, but was not precisely estimated 

with UCODE.

Model Limitations

The model results and interpretations described 

in this report are limited by: (1) assumptions inherent 

to MODFLOW-88 and SEAWAT that may or may not 

be satisfied; (2) difficulty in characterizing small-scale 

heterogeneity that may greatly influence regional 

ground-water flow and saltwater movement; (3) the 

modeling approach; and (4) the inability to quantitatively assess uncertainty in a rigorous fashion for the 

modeling objectives. The primary objectives were to 

identify the predominant mechanisms of saltwater 

intrusion and to estimate the extent of saltwater intrusion in the lower Tamiami aquifer near Bonita Springs.

The partial differential equations approximated 

by MODFLOW-88 and SEAWAT are based on many 

assumptions. Those assumptions relevant to simulating saltwater intrusion in the lower Tamiami aquifer 

near Bonita Springs are that: (1) ground water fully 

saturates the porous media in areas of ground-water 

flow, (2) ground-water flow is described by Darcy's 

law or a variable-density form of Darcy’s law, (3) the 

fluid is incompressible, (4) the standard expression for 

specific storage in a confined aquifer is applicable, 

(5) isothermal conditions prevail, (7) fluid density is a 

linear function of ground-water salinity, and (8) the 

coordinate system is aligned with the principal axis of 

the permeability tensor so that this tensor is diagonal 

for anisotropic media. The natural system probably 

violates some of these assumptions, thus the relevance 

of model results and interpretations to the natural system are limited by the degree to which individual 

assumptions are violated.

The difficulty in characterizing small-scale heterogeneity limits model results, because this heterogeneity may greatly influence regional ground-water 

flow and saltwater movement in the study area. Complete and accurate characterization of heterogeneity in 

the lower Tamiami aquifer, including subsurface structural complexity, variations in hydraulic properties, 

and variations in stresses at multiple scales, was not 

attainable even with the numerous studies documented 

over the years. Thus, heterogeneity at multiple scales 

was averaged, interpolated, or zoned into finite-difference approximations that neglect small-scale particularities, which may be important in the true groundwater flow system in the lower Tamiami aquifer near 

Bonita Springs. 

Clearly, the influence of heterogeneity in porosity at any one particular scale on movement of saltwater is not understood. Effective porosity was assigned a 

uniform value of 30 percent (Reese and Cunningham, 

2000) in the baseline scenario. However, ground-water 

tracer tests suggest fractures and/or high permeability 

zones can give effective porosities of 2 to 4 percent 

(Langevin and others, 1998). Furthermore, some 

researchers have suggested decimeter-scale heterogeneities and preferential flow paths have the dominant 

effect on solute transport at the plume scale, with 

15 percent of the aquifer involved in advective transport while 85 percent of the aquifer serves as a reservoir for essentially immobile solute (Zheng and 

Gorelick, 2001). Collectively, these investigations 

highlight the difficulty in characterizing small-scale 

heterogeneity and suggest the likelihood of a “dual 

porosity” system. Further research into this limitation 

would be useful.

Several assumptions and estimations were 

employed in the modeling approach that could limit 

the reliability of results. These include, but are not limited to: (1) the steady-state assumption, (2) estimating 
Simulation of Saltwater Intrusion near Bonita Springs 61

stresses rather than hydraulic conductivities during 

model calibration, (3) the 7 to 5 ratio of dry- to wetseason months in a typical southwestern Florida water 

year, (4) neglecting the effects of dispersion on simulations of saltwater movement, and (5) initial conditions 

for simulating the movement of saltwater at equilibrium.

Steady-state flow occurs when at any point in 

the flow field, the magnitude and direction of the flow 

velocity are constant with time (Freeze and Cherry, 

1979). Because longer term average values are being 

computed, a steady-state assumption was employed 

while computing the predevelopment distribution of 

saltwater, calibrating to March and September 1996 

conditions, and moving saltwater at equilibrium. The 

steady-state assumption is limiting because transient 

particularities that could be important to saltwater 

movement in the lower Tamiami aquifer near Bonita 

Springs may not be considered. Additionally, using the 

steady-state assumption to calibrate to March and September 1996 conditions is limiting because it is possible the regional ground-water flow system is never at 

true steady state as defined by Freeze and Cherry 

(1979).

Most ground-water models are calibrated by 

adjusting hydraulic conductivity parameters until 

simulated results match observed field conditions. 

The model documented here was calibrated by estimating unmonitored ground-water pumpage and pan 

evaporation multipliers with UCODE. Unmonitored 

ground-water pumpage was estimated because accurately simulated water levels in the lower Tamiami 

aquifer beneath Bonita Springs was most important for 

accurately simulating saltwater movement—a primary 

objective of this study. Hydraulic conductivity was 

correlated with unmonitored ground-water pumpage 

and, therefore, could not be uniquely estimated 

together with UCODE. This was unfortunate because 

the model was more sensitive to hydraulic conductivity parameters, such as canal conductances and the 

vertical hydraulic conductivity of the Tamiami confining beds, than to pan evaporation multipliers. A better 

model fit may have resulted by estimating hydraulic 

conductivity parameters. Additional observations, particularly flow observations, could help obliterate the 

correlation between unknown pumpage and hydraulic 

conductivity parameters. Future modeling efforts 

would likely benefit from using additional flow observations while calibrating.

Previous studies have used a 7 to 5 ratio to represent the number of dry-season months to the number 

of wet-season months during a typical water year in 

southwestern Florida. The 7 to 5 ratio is potentially 

limiting because a plot of average monthly rainfall 

from 69 rainfall stations in southwestern Florida suggests the possibility that an 8 to 4 ratio may be more 

appropriate (fig. 38). The 7 to 5 ratio was used in 

this study to estimate annual average unmonitored 

ground-water pumpage rates, and also was used in the 

baseline scenario and during sensitivity of the baseline 

0

5

10

15

20

25

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

AVERAGE MONTHLY RAINFALL,

IN CENTIMETERS

Figure 38. Average monthly rainfall from selected rainfall stations in southwestern Florida, 1909-99.
62 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

scenario. Using an 8 to 4 ratio would likely have little 

effect on identifying predominant mechanisms of saltwater intrusion and have little effect on sensitivity 

analysis of the baseline scenario. It is probable, however, that using an 8 to 4 ratio would increase estimates of unmonitored ground-water pumpage and 

increase estimates of the extent of saltwater intrusion.

Neglecting the effects of dispersion on simulations of saltwater movement limits the relevance of 

model results to the real nature of ground-water flow 

and saltwater movement in the study area. This is a 

limitation because: (1) the real nature of saltwater 

movement involves mixing, and (2) a dispersive solute-transport simulation suggests dispersion will 

reduce the extent of saltwater intrusion in the lower 

Tamiami aquifer near Bonita Springs. 

In a hypothetical dispersive solute-transport 

simulation, dispersivity in three dimensions was estimated using the relation defined in Gelhar and others 

(1992) and Zheng and Bennett (1995). Longitudinal 

dispersivity was set to 60 m, or one-tenth the horizontal model cell dimension (Gelhar and others, 1992). 

Transverse and vertical dispersivities were set to 6 m, 

or one-tenth the longitudinal dispersivity (Zheng and 

Bennett, 1995). Six hundred years of dispersive and 

advective solute transport was simulated using the 

same modeling approach employed in the baseline 

scenario. In general, dispersion reduced the extent of 

saltwater intrusion. These results are consistent with 

historical studies (Cooper, 1964; Glover, 1964; Henry, 

1964; Kohout, 1964) of the effects of dispersion on the 

saltwater interface, which suggest that including the 

effects of dispersion moves the interface seaward. 

Dispersion was not simulated while solving for 

the predevelopment distribution of saltwater, during 

the baseline scenario, and during sensitivity analysis 

of the baseline scenario due to the length of computer 

run times and uncertainty in estimates of dispersivity. 

Field estimates of dispersivity in the lower Tamiami 

aquifer near Bonita Springs could be useful to constrain the potential effects of dispersion on saltwater 

movement. If computer run times are substantially 

diminished, and if field data can be obtained to set reasonable values of dispersivity at the scale of the model 

grid, then detailed three-dimensional studies of the 

position and behavior of the “toe” of the saltwater 

interface in the lower Tamiami aquifer could be more 

conclusively accomplished.

The lack of initial conditions is a well documented limitation for studies of solute transport in 

ground water. Although maps of salinity distribution 

are available (Knapp and others, 1986; Schmerge, 

2001), these maps are based on field data collected 

over time. Only sparse amounts of salinity data are 

available to instantaneously capture the distribution of 

saltwater at any given time. If salinity is known only at 

a few points within the region to be modeled, and 

assumed initial values are assigned in between, the 

unrealistic flows probably will be calculated by the 

variable-density ground-water flow simulator (Voss, 

1999). A practical solution to this problem was found 

by attempting to solve for initial conditions using predevelopment stresses and boundary conditions 

assumed or extrapolated from contemporary data. This 

approach has several advantages, as outlined in the 

modeling approach section of the report, but still is 

limiting because more accurate solutions to the primary objectives of this study could be obtained if the 

true distribution of freshwater equivalent head and 

ground-water salinity were known. Airborne electromagnetic survey techniques (Fitterman and DeszczPan, 1998), if successfully applied to the water-table 

and lower Tamiami aquifers in southwestern Florida, 

could represent another useful solution to the lack of 

data on initial conditions for saltwater intrusion simulations near Bonita Springs.

Parameter and conceptual uncertainty limit the 

applicability of the modeling results presented here. 

When parameters are changed within reasonable 

ranges, or combinations of parameters are changed 

within reasonable ranges, mechanisms of saltwater 

intrusion become more or less predominant, saltwater 

moves to different extents, and saltwater moves at different rates. This problem is exacerbated by conceptual uncertainty because the ground-water flow system 

could be represented equally well by many different 

boundary conditions, regional distributions of aquifer 

properties, and initial conditions. The effects of this 

uncertainty on the primary study objectives were indirectly addressed through sensitivity analyses. Linear 

95-percent confidence intervals, computed for each 

primary modeling objective, would probably be a 

practical and useful type of analysis to directly address 

parameter uncertainty. The effects of parameter and 

conceptual uncertainty could be directly addressed 

using a composite range of confidence intervals for a 

reasonable set of conceptual models (Poeter, 2001). 

However, these analyses were considered beyond the 

scope of this investigation. 
Summary 63

SUMMARY

Saltwater intrusion is a concern near Bonita 

Springs in southwestern Florida where the average 

annual potentiometric surface in some areas of the 

lower Tamiami aquifer is below sea level. Field data 

were analyzed, and model simulations were performed 

to: (1) spatially quantify modern and seasonal stresses, 

(2) help identify potential mechanisms of saltwater 

intrusion, (3) and estimate the potential extent of saltwater intrusion in the lower Tamiami aquifer near 

Bonita Springs.

MODFLOW and the inverse modeling routine 

UCODE were used to spatially quantify modern and 

seasonal stresses in 1996 by calibrating a constantdensity ground-water flow model. The model was calibrated by assuming hydraulic conductivity parameters 

were accurate and by estimating unmonitored groundwater pumpage and pan-evaporation multipliers with 

UCODE. Additionally, uncertainty in estimates of 

these parameters was quantified through 95-percent 

confidence intervals. These confidence intervals indicate more uncertainty or less reliability in the estimates of unmonitored ground-water pumpage than 

estimates of pan-evaporation multipliers due to the 

nature and distribution of observations used during 

calibration. Comparison of simulated water levels, 

streamflows, and net recharge with field data demonstrate the adequacy of the simulated water budget. 

Net recharge and unmonitored ground-water 

pumpage are two components of the water budget of 

interest to water managers. Using modeling results for 

1996, average net recharge to the water-table aquifer 

was estimated to range between 0 and 38 cm/yr, and 

unmonitored ground-water pumpage from the lower 

Tamiami aquifer was estimated to be 52,000 m3

/d. Net 

recharge less than zero occurred over less than 5 percent of the active model domain due to the close proximity of the water table to land surface. The average 

estimated rate of unmonitored ground-water pumpage 

in 1996 from the lower Tamiami aquifer (52,000 m3

/d) 

was almost as much as the monitored ground-water 

pumpage in 1996 from the water-table and lower 

Tamiami aquifers (59,000 m3

/d) in the active model 

domain.

Potential mechanisms of saltwater intrusion 

into the lower Tamiami aquifer near Bonita Springs 

include: (1) lateral inland movement of the freshwatersaltwater interface from the southwestern coast of 

Florida; (2) upward leakage from deeper, saline 

water-bearing zones through natural upwelling and 

upconing, both of which could occur as diffuse upward 

flow through semiconfining layers, conduit flow 

through karst features, or pipe flow through leaky artesian wells; (3) downward leakage of saltwater from 

surface-water channels; and (4) movement of 

unflushed pockets of relict seawater. Of the many 

potential mechanisms, field data and variable-density 

ground-water flow simulations suggest upconing is 

probably of utmost concern, and lateral encroachment 

is probably of second most concern in the lower Tamiami aquifer beneath Bonita Springs. This interpretation is uncertain, however, because the predominance 

of leaky wells with connection to deeper, more saline, 

and more pressurized aquifers was difficult to 

establish.

The potential extent of saltwater intrusion was 

estimated using a variable-density ground-water flow 

simulator and 1996 seasonal stresses. The potential 

extent of saltwater intrusion in the lower Tamiami 

aquifer beneath Bonita Springs is about 100 km2

 and 

70,000 hectare-meters. The volumetric extent of saltwater intrusion was most sensitive to changes in 

recharge, ground-water pumpage, sea level, salinity of 

the Gulf of Mexico, and the potentiometric surface of 

the sandstone aquifer, respectively, as indicated by prediction-scaled sensitivities. Future efforts at estimating 

the extent of saltwater intrusion in this area would 

likely benefit from improved representation of these 

hydrologic stresses and boundary conditions.

Composite-scaled sensitivities that were computed while calibrating the model were compared with 

prediction-scaled sensitivities computed for the potential extent of saltwater intrusion predicted by the 

model. Composite-scaled sensitivities suggested 

observed conditions provided the most information for 

estimating net recharge. Prediction-scaled sensitivities 

suggested net recharge is the most important parameter for estimating the extent of saltwater intrusion in 

the lower Tamiami aquifer beneath Bonita Springs. 

This consistency was encouraging and supports the 

idea that calibrated ground-water flow models can be 

used to adequately simulate saltwater movement.
64 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

SELECTED REFERENCES

Albury, Carl, Langevin, C.D., and Stewart, M.T., 1998, 

Determining the feasibility for wetland rehydration: 

Simulation of application rates and transport of 

reclaimed water: Hydrogeology Laboratory, University 

of South Florida, Tampa, 11 p. 

Anderson, M.P., and Woessner, W.W., 1992, Applied 

groundwater modeling: London, Academic Press, 

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Appendix

Monitoring Stations Used for this Study

[Station type: C, chloride; E, pan-evapotranspiration; R, rainfall; WL, water level. Data source: DBHYDRO, South 

Florida Water Management District hydrometorologic database; NWIS, U.S. Geological Survey National Water 

Information System]
68 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida
Appendix 69

Appendix. Monitoring stations used in this study

Station

identification

Station 

type Latitude Longitude

Data

source

Station

identification

Station 

type Latitude Longitude

Data

source

951EXT_R R 261809 814118 DBHYDRO C-496 WL 260112 812438 NWIS

ALVA FAR_R R 264245 813747 DBHYDRO C-496 C 260112 812438 NWIS

ASGROW R 261616 814229 DBHYDRO C-503 C 261742 812353 NWIS

BARRON_R R 255801 812059 DBHYDRO C-506A WL 261234 814801 NWIS

BAY WEST_R R 261630 814208 DBHYDRO C-506A C 261234 814801 NWIS

BCBNAPLE_R R 261331 814829 DBHYDRO C-515 C 261347 814801 NWIS

BONITA S_R R 262001 814459 DBHYDRO C-516 C 261157 814757 NWIS

C-39 C 254859 812315 NWIS C-524 C 260949 814832 NWIS

C-54 WL 261019 805301 NWIS C-525 C 261003 814836 NWIS

C-54 C 261019 805301 NWIS C-526 C 261019 814840 NWIS

C-54_R R 261019 805301 DBHYDRO C-527 C 261049 814847 NWIS

C-123 C 261004 814757 NWIS C-528 C 261201 814829 NWIS

C-130 C 260903 814803 NWIS C-531 WL 262900 812729 NWIS

C-131 WL 262522 811618 NWIS C-531 C 262900 812729 NWIS

C-131 C 262522 811618 NWIS C-532 C 262900 812729 NWIS

C-161 C 261024 814706 NWIS C-575 C 261318 814804 NWIS

C-175 C 260913 814345 NWIS C-598 WL 261418 813053 NWIS

C-258 C 262505 812458 NWIS C-598 C 261418 813053 NWIS

C-269 C 255701 812744 NWIS C-599 C 260631 814113 NWIS

C-296 C 260641 812042 NWIS C-600 WL 260550 814418 NWIS

C-296_R R 260641 812042 DBHYDRO C-600 C 260550 814418 NWIS

C-298 C 262508 812351 NWIS C-684 C 261741 812353 NWIS

C-303 C 261622 814122 NWIS C-687 C 262555 812837 NWIS

C-304 C 261636 813612 NWIS C-688 C 261803 813547 NWIS

C-308 C 260920 811558 NWIS C-689 C 261741 812353 NWIS

C-311 C 255431 812209 NWIS C-690 WL 260630 813234 NWIS

C-321 C 261436 814724 NWIS C-690 C 260630 813234 NWIS

C-363 C 262556 812424 NWIS C-948 C 261348 813516 NWIS

C-384 C 261621 814506 NWIS C-951 WL 261348 813511 NWIS

C-391 WL 261125 814702 NWIS C-951 C 261348 813511 NWIS

C-392 WL 261125 814700 NWIS C-953 WL 261348 813511 NWIS

C-392 C 261125 814700 NWIS C-953 C 261348 813511 NWIS

C-409 C 261025 814800 NWIS C-955 C 261722 813512 NWIS

C-409A WL 261025 814800 NWIS C-956 C 261344 813847 NWIS

C-409A C 261025 814800 NWIS C-963 C 262122 813554 NWIS

C-424 C 261525 814803 NWIS C-965 C 262137 812041 NWIS

C-430 C 261147 814606 NWIS C-966 C 262137 812041 NWIS

C-445 C 255135 812305 NWIS C-967 C 260541 814305 NWIS

C-446 C 260449 814115 NWIS C-968 WL 260335 813915 NWIS

C-458 C 261402 814613 NWIS C-968 C 260335 813915 NWIS

C-459 C 261404 814706 NWIS C-969 WL 260231 814013 NWIS

C-460 WL 261406 814654 NWIS C-969 C 260238 814014 NWIS

C-460 C 261406 814654 NWIS C-970 C 261722 813512 NWIS

C-462 WL 262725 812606 NWIS C-971 C 261722 813512 NWIS

C-462 C 262725 812606 NWIS C-972 C 260844 813241 NWIS

C-472A C 260926 814751 NWIS C-973 C 260844 813241 NWIS

C-474 C 261115 814822 NWIS C-974 C 260942 813241 NWIS

C-474A C 261115 814822 NWIS C-975 C 260305 813913 NWIS

C-489 C 261303 814738 NWIS C-976 C 260916 813858 NWIS

C-490 C 261244 814802 NWIS C-977 C 260916 813858 NWIS

C-491 C 261118 814800 NWIS C-978 C 262122 813554 NWIS

C-492 WL 262229 813618 NWIS C-979 C 262122 813554 NWIS

C-492 C 262229 813618 NWIS C-980 C 261344 813847 NWIS

C-495 C 255749 811817 NWIS C-981 C 262159 812833 NWIS
70 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

C-982 C 262159 812833 NWIS C-1079 C 262159 812823 NWIS

C-983 C 262159 812833 NWIS C-1080 C 262229 813618 NWIS

C-984 C 261734 812854 NWIS C-1081 C 260248 814121 NWIS

C-985 C 261734 812854 NWIS C-1082 C 261805 814733 NWIS

C-986 C 261201 812048 NWIS C-1083 WL 261856 814719 NWIS

C-987 C 260310 812725 NWIS CAPECOR1_R R 263524 820054 DBHYDRO

C-988 WL 261445 812848 NWIS CAPECOR2_R R 263445 815748 DBHYDRO

C-988 C 261445 812848 NWIS CAPTIVA_R R 263201 821059 DBHYDRO

C-989 WL 261734 812854 NWIS CARIBBEA_E E 261001 814659 DBHYDRO

C-989 C 261734 812854 NWIS CCWTP_R R 261601 814659 DBHYDRO

C-995 C 255704 812137 NWIS CCWWTP_R R 261606 814712 DBHYDRO

C-996 C 260909 814111 NWIS CCWWTP2_R R 261431 814029 DBHYDRO

C-997 WL 252751 802833 NWIS COCO1_R R 261622 814647 DBHYDRO

C-997 C 261530 814120 NWIS COCOH.WB_R R 261622 814550 DBHYDRO

C-998 C 261621 814501 NWIS COLGOV_R R 260747 814545 DBHYDRO

C-999 C 261509 814848 NWIS COLLICTY R 261011 814116 DBHYDRO

C-1000 C 261104 814748 NWIS COLLIER_R R 260924 813929 DBHYDRO

C-1001 C 261117 814802 NWIS COLLISEM R 255926 813529 DBHYDRO

C-1002 C 261242 814653 NWIS COPELAND_R R 255701 812136 DBHYDRO

C-1003 C 261437 814802 NWIS CORK.CP_R R 262427 813658 DBHYDRO

C-1004 WL 261621 814643 NWIS CORK.HQ_E R 262301 813459 DBHYDRO

C-1004 C 261621 814643 NWIS CORK.HQ_R R 262301 813459 DBHYDRO

C-1026 C 261234 814801 NWIS CORK.LCI_R R 262431 813417 DBHYDRO

C-1051 C 261025 814637 NWIS CORK.SD_R R 262147 813826 DBHYDRO

C-1052 C 260920 814604 NWIS CORK.TOW_R R 263123 813459 DBHYDRO

C-1053 C 260920 814604 NWIS CORK_R R 262520 813443 DBHYDRO

C-1054 C 261128 814609 NWIS CORKISL R 262152 813531 DBHYDRO

C-1055 C 261212 814412 NWIS DANHP_R R 255843 812851 DBHYDRO

C-1056 C 261458 814608 NWIS DUDA.NAP_R R 260151 813911 DBHYDRO

C-1057 C 261538 814611 NWIS EAGLECRK R 260307 814224 DBHYDRO

C-1058 C 261538 814611 NWIS ESTERO T_R R 262826 815017 DBHYDRO

C-1059 C 261605 814808 NWIS EVERGL 2_R R 255043 812313 DBHYDRO

C-1060 C 261500 814803 NWIS FAKA_R R 255738 813034 DBHYDRO

C-1061 C 261312 814800 NWIS FAKAHAT_R R 255845 812429 DBHYDRO

C-1062 C 260926 814750 NWIS FAKAHATC_R R 261002 812140 DBHYDRO

C-1063 C 260138 813758 NWIS FORT MEY_R R 263501 815159 DBHYDRO

C-1064 WL 260138 813758 NWIS FPWX R 262557 814324 DBHYDRO

C-1064 C 260138 813758 NWIS FT MEYER_R R 263451 815151 DBHYDRO

C-1065 C 255638 812813 NWIS GOLD.FS_R R 261058 814216 DBHYDRO

C-1066 C 255638 812813 NWIS GOLD.W1_R R 261004 814604 DBHYDRO

C-1067 C 260315 813230 NWIS GOLD.WP_R R 261101 814159 DBHYDRO

C-1068 C 260315 813230 NWIS GOLD.WP2_R R 261001 814214 DBHYDRO

C-1069 C 260814 812142 NWIS GOLD.WP3_E E 261001 814159 DBHYDRO

C-1070 C 260814 812142 NWIS GOLD75 R 260929 813114 DBHYDRO

C-1071 WL 261824 811718 NWIS GOLDFS2 R 261342 813755 DBHYDRO

C-1071 C 261824 811718 NWIS GORDON_R R 261022 814705 DBHYDRO

C-1072 WL 261824 811718 NWIS HENDER_R R 260559 814112 DBHYDRO

C-1072 C 261824 811718 NWIS IMMOKA 2_R R 262426 812459 DBHYDRO

C-1073 C 261741 812353 NWIS IMMOKA 3_R R 262741 812614 DBHYDRO

C-1074 WL 262520 811620 NWIS IMMOKALE_R R 262335 812425 DBHYDRO

C-1074 C 262520 811620 NWIS JUNGLE L_R R 261005 814723 DBHYDRO

C-1075 C 262823 812131 NWIS KANTORS_R R 261129 814141 DBHYDRO

C-1076 C 262823 812131 NWIS L TRAFFO_R R 262601 812859 DBHYDRO

C-1077 C 262823 812131 NWIS L.B.MINO_R R 264436 813557 DBHYDRO

C-1078 C 262559 812704 NWIS L-246 C 263803 814934 NWIS

Appendix. Monitoring stations used in this study (Continued)

Station

identification

Station 

type Latitude Longitude

Data

source

Station

identification

Station 

type Latitude Longitude

Data

source
Appendix 71

L-331 C 263126 815117 NWIS L-1156 C 263316 815241 NWIS

L-345 C 262035 814647 NWIS L-1403 WL 262550 820352 NWIS

L-346 C 262035 814646 NWIS L-1403 C 262550 820352 NWIS

L-351 C 262911 820035 NWIS L-1418 WL 263631 813752 NWIS

L-352 C 262909 820037 NWIS L-1418 C 263631 813752 NWIS

L-357 C 263059 815607 NWIS L-1456 C 262623 820219 NWIS

L-581 WL 263533 815921 NWIS L-1457 C 262623 820219 NWIS

L-581 C 263533 815921 NWIS L-1598 C 263234 815502 NWIS

L-585 C 262709 820053 NWIS L-1625 C 263330 813942 NWIS

L-588 C 262539 820456 NWIS L-1634 C 262436 815350 NWIS

L-590 C 262549 820509 NWIS L-1635 C 262436 815349 NWIS

L-652 C 264102 814429 NWIS L-1691 WL 262043 814549 NWIS

L-721 C 264154 820222 NWIS L-1691 C 262043 814549 NWIS

L-726 C 264426 814539 NWIS L-1853 C 262707 814353 NWIS

L-727 WL 263851 813653 NWIS L-1907 C 264309 814059 NWIS

L-727 C 263851 813653 NWIS L-1963 C 263345 813616 NWIS

L-728 C 263713 814611 NWIS L-1964 C 263345 813616 NWIS

L-729 WL 263336 813942 NWIS L-1965 C 263354 813357 NWIS

L-729 C 263336 813942 NWIS L-1968 C 263808 814302 NWIS

L-730 C 263138 815458 NWIS L-1973 C 263719 814849 NWIS

L-730 WL 263138 815458 NWIS L-1974 C 263719 814849 NWIS

L-731 WL 262704 813401 NWIS L-1975 C 264400 814246 NWIS

L-731 C 262704 813401 NWIS L-1976 C 264400 814246 NWIS

L-735 C 262840 815030 NWIS L-1977 C 264321 813656 NWIS

L-738 C 262023 814641 NWIS L-1978 C 264321 813656 NWIS

L-739 C 262658 814434 NWIS L-1983 C 263042 814330 NWIS

L-741 C 262553 814856 NWIS L-1984 C 262714 814145 NWIS

L-742 WL 263324 815223 NWIS L-1985 C 262714 814146 NWIS

L-742 C 263324 815223 NWIS L-1992 C 263354 813357 NWIS

L-781 WL 263835 820052 NWIS L-1993 WL 263252 814527 NWIS

L-781 C 263835 820052 NWIS L-1993 C 263252 814527 NWIS

L-954 WL 263904 815503 NWIS L-1994 WL 263252 814527 NWIS

L-954 C 263904 815503 NWIS L-1994 C 263252 814527 NWIS

L-1058 C 263815 820206 NWIS L-1995 WL 263252 814527 NWIS

L-1059 C 264518 820220 NWIS L-1995 C 263252 814527 NWIS

L-1089 C 263125 815213 NWIS L-1996 WL 261955 814100 NWIS

L-1099 C 264054 815631 NWIS L-1996 C 261955 814100 NWIS

L-1106 C 264055 815925 NWIS L-1997 WL 261955 814100 NWIS

L-1107 C 264147 815922 NWIS L-1997 C 261955 814100 NWIS

L-1108 C 264145 815825 NWIS L-1998 WL 263042 814330 NWIS

L-1109 C 264056 815830 NWIS L-1998 C 263042 814330 NWIS

L-1110 C 264242 815823 NWIS L-1999 C 263042 814330 NWIS

L-1111 C 264148 815626 NWIS L-2186 WL 263345 813616 NWIS

L-1113 C 264121 820220 NWIS L-2186 C 263345 813616 NWIS

L-1114 C 263721 815730 NWIS L-2187 C 263951 813553 NWIS

L-1115 C 263906 815727 NWIS L-2190 C 264145 815202 NWIS

L-1116 C 263634 820026 NWIS L-2191 C 264145 815202 NWIS

L-1117 C 263439 815631 NWIS L-2192 C 262700 813824 NWIS

L-1121 C 263328 815119 NWIS L-2194 WL 261958 814321 NWIS

L-1124 C 263247 815314 NWIS L-2194 C 261958 814321 NWIS

L-1129 C 263324 815352 NWIS L-2195 WL 261958 814321 NWIS

L-1136 C 263533 815921 NWIS L-2195 C 261958 814321 NWIS

L-1137 WL 263951 813553 NWIS L-2198 C 261955 814321 NWIS

L-1137 C 263951 813553 NWIS L-2200 C 264330 813403 NWIS

L-1138 C 262704 813401 NWIS L-2202 C 264330 813403 NWIS

Appendix. Monitoring stations used in this study (Continued)

Station

identification

Station 

type Latitude Longitude

Data

source

Station

identification

Station 

type Latitude Longitude

Data

source
72 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida

L-2204 C 263330 813942 NWIS L-3213 C 263358 815755 NWIS

L-2212 C 262832 815758 NWIS L-3214 C 263956 820830 NWIS

L-2215 C 263128 813515 NWIS L-3215 C 263118 820509 NWIS

L-2216 C 264609 814540 NWIS L-4820 C 264054 815724 NWIS

L-2217 C 264609 814540 NWIS L-5648 C 263250 814743 NWIS

L-2244 C 263243 815720 NWIS L-5649 WL 262935 814957 NWIS

L-2292 C 263719 814849 NWIS L-5649 C 262935 814957 NWIS

L-2295 C 262553 814856 NWIS L-5664 C 262515 813933 NWIS

L-2308 C 262553 814856 NWIS L-5665 C 262515 813933 NWIS

L-2310 C 262023 814641 NWIS L-5666 C 262514 814328 NWIS

L-2311 C 263345 813616 NWIS L-5667 C 262514 814325 NWIS

L-2313 C 262704 813401 NWIS L-5668 C 262514 814717 NWIS

L-2315 C 263005 821116 NWIS L-5669 C 262512 814717 NWIS

L-2328 C 264609 814540 NWIS L-5672 C 262332 813831 NWIS

L-2341 C 264518 815131 NWIS L-5673 C 262332 813831 NWIS

L-2434 WL 263527 820101 NWIS L-5707 C 263853 815147 NWIS

L-2434 C 263527 820101 NWIS L-5720 C 263250 814743 NWIS

L-2435 C 263407 815559 NWIS L-5721 C 262935 814957 NWIS

L-2524 C 262623 820743 NWIS L-5722 C 262103 814643 NWIS

L-2525 C 263118 820509 NWIS L-5723 C 262103 814643 NWIS

L-2526 C 264518 820220 NWIS L-5724 C 261947 814902 NWIS

L-2527 C 263956 820830 NWIS L-5725 C 261947 814902 NWIS

L-2528 C 263908 815926 NWIS L-5726 C 261900 814818 NWIS

L-2529 C 262914 815624 NWIS L-5727 C 261900 814818 NWIS

L-2530 C 264309 814053 NWIS LEHIGH 1_R R 263625 813859 DBHYDRO

L-2531 C 264428 813625 NWIS LEHIGH 2_R R 263907 813935 DBHYDRO

L-2549 C 263956 820830 NWIS LEHIGH 3_R R 263726 813403 DBHYDRO

L-2550 WL 262712 814137 NWIS LEHIGH 4_R R 263325 813623 DBHYDRO

L-2640 C 263814 815527 NWIS LEHIGH 5_R R 263616 814218 DBHYDRO

L-2641 C 263534 815733 NWIS LEHIGH 6_R R 263701 814359 DBHYDRO

L-2642 C 263258 815856 NWIS LEHIGH E_R R 263301 814259 DBHYDRO

L-2643 C 263254 820141 NWIS LEHIGH W_R R 263626 813859 DBHYDRO

L-2644 WL 263441 820219 NWIS LEHIGH_R R 263625 813859 DBHYDRO

L-2644 C 263441 820219 NWIS MARCO FI_R R 255548 814201 DBHYDRO

L-2645 C 263744 820411 NWIS MARCO TO_R R 260001 813459 DBHYDRO

L-2646 C 264538 815521 NWIS MILES 2_R R 261145 812045 DBHYDRO

L-2700 C 264003 820127 NWIS MILES 3_E E 261100 812600 DBHYDRO

L-2701 WL 263820 815857 NWIS MILES CI_R R 261104 812047 DBHYDRO

L-2701 C 263820 815857 NWIS MONROE T_R R 255131 810624 DBHYDRO

L-2702 WL 263622 815636 NWIS NAPLES C_R R 260738 814459 DBHYDRO

L-2702 C 263622 815636 NWIS NAPLES T_R R 260914 814532 DBHYDRO

L-2703 WL 263358 815755 NWIS NAPLES_R R 261005 814723 DBHYDRO

L-2703 C 263358 815755 NWIS NNAPFS42 R 261620 814332 DBHYDRO

L-2820 C 263956 820830 NWIS NP-EVC R 255112 812249 DBHYDRO

L-2821 C 263118 820509 NWIS NP-OAS R 255127 810205 DBHYDRO

L-3203 C 263814 815527 NWIS OASIS R 255101 810159 DBHYDRO

L-3204 C 263534 815733 NWIS RACOON PT R 255801 811859 DBHYDRO

L-3205 C 263258 815856 NWIS ROYAL HA_R R 255926 813529 DBHYDRO

L-3206 C 263254 820141 NWIS S79_R R 264326 814154 DBHYDRO

L-3207 C 263441 820219 NWIS SDS_R R 261630 814208 DBHYDRO

L-3208 C 263744 820411 NWIS SILVER S_R R 261749 812618 DBHYDRO

L-3209 C 264538 815521 NWIS SITE #1_R R 260900 814100 DBHYDRO

L-3210 C 264003 820127 NWIS SITE #2_R R 255300 811800 DBHYDRO

L-3211 C 263820 815857 NWIS SITE1_R R 261643 813341 DBHYDRO

L-3212 C 263622 815636 NWIS SITE2_R R 261149 814016 DBHYDRO

Appendix. Monitoring stations used in this study (Continued)

Station

identification

Station 

type Latitude Longitude

Data

source

Station

identification

Station 

type Latitude Longitude

Data

source
Appendix 73

SITE3_R R 260343 813124 DBHYDRO SR_R R 263501 815159 DBHYDRO

SITE4 2_R R 261616 813144 DBHYDRO STEPHAN R 261011 814116 DBHYDRO

SIX L.7_R R 260114 813723 DBHYDRO TAMIAMI_R R 255311 811529 DBHYDRO

SLEE_R R 264145 814637 DBHYDRO USDA IMM_R R 262741 812614 DBHYDRO

VICTORIA_R R 261541 814614 DBHYDRO

Appendix. Monitoring stations used in this study (Continued)

Station

identification

Station 

type Latitude Longitude

Data

source

Station

identification

Station 

type Latitude Longitude

Data

source
74 Potential for Saltwater Intrusion into the Lower Tamiami Aquifer Near Bonita Springs, Southwestern Florida